Immaterium

"The warp is a strange and terrible place. You might as well throw a traveller into a sea of sharks and tell him to swim home as send him through the warp unprotected. Better it is not to let common man travel through the stars. Better still, let him not know such a thing is feasible."

- –Fra Safrane, 5th aide to Navigator De’el.

The Immaterium (also referred to as the Empyrean, the Aether, the Sea of Souls, the Realm of Chaos, Warpspace or most commonly, the Warp) is an alternate dimension of purely psychic energy that echoes and underlies the familiar four dimensions of the material universe. It is the source of all psychic powers and known instances of so-called "sorcery" or "magic" as well as the home dimension of the Chaos Gods and their myriad daemonic servants. In fact, the terms "Chaos" and "the Warp" are often used interchangeably by those aware of their existence within the Imperium of Man. Superficially, the Immaterium is Mankind's solution to the problem of faster-than-light travel, an equivalent to the Star Wars universe's dimension of hyperspace. This function as a faster-than-light medium for space travel is achieved because the Immaterium is a domain of pure psychic energy, with spacecraft navigating between its currents, as in an ocean. The psychic energy that makes up the Immaterium is believed to be the direct result of the existence of sentience in the universe, in particular the intelligent species of the Milky Way Galaxy. Considered to be a dark reflection of the material universe, the Warp is an ocean of chaotic psychic energy, raw emotion given physical form. Stirred by emotion and action, the Immaterium is the true Realm of Chaos, home to the Dark Gods who comprise the Ruinous Powers and their legions of daemonic followers. The Immaterium is also rumoured by many cultures, human and xenos alike, to be the final resting place of the spirits of the dead, and therefore can be considered the "Underworld" of the Warhammer 40,000 universe.

Warp Travel
The realm of the Immaterium, while a potentially deadly environment for beings of the material universe, can be used for faster-than-light communication and non-Euclidean travel between two points in realspace. Vast amounts of energy are required for a vessel to enter the Immaterium from the physical world, and while within it, they are often at the mercy of the currents and eddies that make up the chaotic dimension. Warp travel is not to be taken lightly, and the dangers range from hull damage to daemonic possession. Entire fleets have entered the Warp and never emerged, lost to the infamous Chaos spawn and the corruption and insanity that follows. Time also proceeds a great deal slower in the Warp than in the material universe, creating a form of stasis. In real time, journeys can take months or even years, though to the crew and passengers only weeks seem to pass in the Warp. The timing itself is unpredictable due to the very nature of the Immaterium, and occasionally a fleet of reinforcements will emerge from the Immaterium to find that the war they were sent to fight in has long been lost, or not yet even started!

Necessities of Warp Travel
Several things are necessary for travel within the Immaterium. Primary among these is the ancient device capable of producing what is known as a Gellar Field, a technologically-generated psychic ward that protects the starship by generating a bubble of realspace around the vessel. Without this arcane techno-magic, the souls of those travelling within the Warp would be constantly exposed to the many daemons and psychic creatures that inhabit that twisted realm. For starship jumps any longer than a handful of light-years, a member of the mutant psyker breed known as Navigators must be aboard. These mutant humans are able to "see" the currents of the Immaterium with their psychic "third eye", and are able to guide a starship to its intended destination with much greater accuracy than normal humans or even the aid of Cogitators (computers). However, for a Navigator to fulfil this role, he needs an unchanging astrocartographic reference point, which is provided by the Astronomican.

Dangers of Warp Travel
The Immaterium is far from a safe place to travel through. Great currents and storms can blow starships into uncharted areas of the galaxy and make travel through certain regions impossible. Warp Storms that last for centuries can isolate sectors and leave fleets becalmed within the material universe. Even worse, starships could find themselves becalmed within the Immaterium itself, a terrible fate for its passengers as they become playthings for the dark creatures that inhabit that diabolical realm. Sometimes a ship will emerge from Warp space centuries after it left yet its passengers have experienced only a few days onboard, while many more vanish without trace.

Fast-Flowing River Analogy
A common explanation of the Warp's mechanics for faster-than-light travel likens it to a fast-flowing river, where the material world is the river's bank; by dropping a leaf into the river, it can travel faster by bobbing along within the river than someone walking along the bank, but it is at the mercy of rapids (Warp Storms), rocky areas (daemons, Enslavers, other dangerous Warp entities) and flooding (Warp Rifts). While this is a useful metaphor in understanding the nature of the Empyrean, in truth the Warp is far more complex in its non-Euclidean, multidimensional movements than a linear stream. The Warp moves in endlessly unpredictable, chaotic and convoluted patterns that none can truly predict with any great degree of accuracy.

Realm of Chaos
The Ruinous Powers of Chaos and their daemonic servants are known to inhabit the portion of the Immaterium often called the Realm of Chaos. Daemons are only able to exist within the Immaterium, unless certain conditions are met within the material realm. The Immaterium is what empowers the daemonic and mortal servants alike of the four Dark Gods -- Khorne the Blood God, Slaanesh the Prince of Pleasure, Tzeentch the Architect of Fate, and Nurgle the Lord of Decay -- and their followers will undertake any action to increase this power and their standing with their unholy masters. Each of the Chaos Gods maintains his own kingdom within the Realm of Chaos, the nature of which matches his own personality and interests exactly. The Realm of Chaos is marked by nearly constant warfare and strife between the daemonic servants of the Ruinous Powers, who constantly scheme and battle against once another in an eternal struggle for dominance that can never be won. The Chaos Gods and their servants are consumed by this struggle and in truth pay very little attention to the doings of realspace, save when the actions of mortals either present a threat to all of the Chaos Gods' continued activities or an opportunity for the Ruinous Powers to enhance their position against one another in their eternal conflict.

Formless Wastes
The Formless Wastes, also known as the Chaotic Abyss and the Land of Lost Souls, are those regions of the Immaterium that do not fall under the dominion of one of the four Chaos Gods. Much of its landscape is simply a terrain of pure chaos that is forming and reforming constantly based on the psychic currents roiling the minds of the sentient mortals of the Milky Way Galaxy as well as the psychic currents of the Realm of Chaos. The Formless Wastes are a place where the laws of physics hold no sway. Within the Formless Wastes sit rivers of tar that flow through petrified woodlands beneath crimson skies. Great stairways ascend into the heavens where they connect back on themselves in an endless loop. These celestial stairways sit alongside castles crafted from bone and fortresses of ichor while pillars of fire burn on the horizon. Every dream, nightmare and vision ever had by one of the sentient races of the galaxy has a representation within the Formless Wastes. The Wastes are also the home of the daemonic Furies who watch for any prey, mortal or daemon, that comes within their reach.

Those Greater Daemons and Daemon Princes that are strong enough to maintain a level of psychic control over their environment within the Warp also make their homes within the Wastes. These islands of horrific stability stand out within this chaotic wilderness with some only appearing for an instant whilst others are as old as the Ruinous Powers themselves. Each of these sub-realms is a small extension of the Realm of Chaos that embodies the whims of its creator and has a shrine or temple dedicated to its beliefs. Some are even populated by daemons born from the mind of the Greater Daemon or Daemon Prince, though these entities are usually short-lived as they tend to be consumed by the Furies that populate the Formless Wastes and are much weaker than the daemons of the four major Chaos Gods.

The minor Chaos Gods are also believed to sometimes maintain their demesnes within the Formless Wastes. Also within the Formless Wastes of the Immaterium resides the dark daemonic foundry known as the Forge of Souls.

Psychic Realm
The Immaterium is a dimension of the spiritual, created, sustained and influenced by intelligent mortal emotions and actions in the physical universe. The mind of every sentient being in the material universe save the Necrons and the C'tan leaves a psychic imprint within the Immaterium, and although the signature of one mind is almost insignificant in the energy or influence it generates, when the imprints of an entire intelligent race of billions upon billions of individuals are combined they have a huge impact on the very nature and shape of the Warp (this seems to imply that the Immaterium is not chaotic by nature, but has been made so by the chaos and war that defines so much of the material realm). When an emotion or faith in an entity or concept grows strong enough, it actually becomes one of the denizens of the Warp. Many of these beings vanish when the emotion or beliefs that create them pass, but the strongest and most archetypal can become self-perpetuating, spreading the thoughts and events that grant them form and power and even interacting with the material world in a way that further strengthens the beleifs and attitudes that give them existence. The strongest of these beings become the gods and daemons associated with a specific group or culture, and almost every single intelligent race or force of will has its universal concepts or religious figures personified in some way by the Warp, be they Chaos, Eldar, Ork, or human.

Psykers
The extrasensory, seemingly magical abilities of the psychically-gifted individuals known in the Imperium of Man as psykers are powered by the energies of the Warp, and when such power is used, the daemons and creatures of the Immaterium are drawn to the psyker's mind like moths to a flame. Those humans trained in the use of the psychic arts are able to resist such incursion to a degree, but the greatest fear of the Inquisition is the fact that an untrained human psyker of sufficient power can become the target of daemonic possession -- and a gateway for the intrusion of the daemonic servants of the Ruinous Powers into the physical world.

Realm of the Dead
Many cultures of the Milky Way Galaxy, both human and xenos, believe that the Immaterium is the final resting place for the souls or spirits of the dead. Certainly there is some truth to this notion. The Eldar believe that upon their deaths, their souls travel into the Warp where they are immediately devoured by their race's ancient enemy, the Chaos God Slaanesh. Because the Eldar's hedonistic and intensely psychic nature was responsible for bringing Slaanesh into existence within the Empyrean following the Fall of the Eldar sometime between the 29th and 30th Millennia, it can be said that Eldar souls have a form of "psychic gradient" in which they are immediately consumed by the hunger of Slaanesh upon the death of their physical bodies if protective measures are not taken. For the Eldar of the Craftworlds, this protection takes the form of the Spirit Stone worn by every one of their number which immediately absorbs their soul's psychic energy upon death so that it can later be deposited for safekeeping within their Craftworld's Infinity Circuit. There it will exist for all eternity as part of a gestalt collective mind of the dead. The Dark Eldar take a predictably more savage course. They are partially protected by their existence within the Labyrinthine Dimension of the Webway from consumption by Slaanesh, but "She Who Thirsts" will eventually consume even a Dark Eldar's soul unless he draws psychic energy to protect himself from the psychic agony of others. It is for this reason that the Dark Eldar are such malevolent creatures, seeking out an endless supply of slaves whose torture they not only enjoy but which actually keeps their twisted souls intact.

Amongst the mortal servants of the Chaos Gods, there is no doubt: when a mortal dies in realspace, his soul is immediately swept into the Warp at the moment his physical body dies, where it will be devoured by the daemons and other Warp entities or made into their pitible plaything for all eternity. The only salvation from this grim fate is to give oneself over body and soul to the worship and service of the Ruinous Powers either collectively as Chaos Undivided or individually as the Dark Gods may grant their more successful and powerful followers a favoured place within the Realm of Chaos upon their deaths. The most successful devotees of Chaos can even look forward to an existence as a Daemon Prince, a position of guaranteed immortality and control over one's own small demesne or Daemon World within the Realm of Chaos or a Warp Rift like the Eye of Terror.

The devotees of many variants of the Imperium of Man's Imperial Cult also believe that when a person dies, his soul is transported to the Warp. But a devout worshipper of the God-Emperor of Mankind holds that the Emperor protects his faithful within the Warp from all entities that might seek to harm them and gathers the souls of the faithful to himself. Only someone who has not been faithful to the Emperor and carried out His will or obeyed His commandments is at risk of being devoured by daemons of the Warp or otherwise punished within the Empyrean. In this way, the devotees of the Imperial Cult see the Warp as an analogue of the old Judeo-Christian concept of Heaven and Hell -- the Warp is Hell for those who have been unfaithful to the God-Emperor, but can become a form of Heaven for those who have been true to His will. Whether there is any truth to this belief is unknown, but the psychic nature of the Empyrean means that the collective and strongly-held beliefs of mortals in realspace tend to take on a reality of their own within the Warp. And there are a great many human beings in the galaxy, and a great many of them are fervent, even fanatical believers in the Imperial Creed. What is known with certainty is that those men and women of the Imperium who have displayed unshakeable faith in their concept of the God-Emperor have sometimes been able to enact feats that can only be considered miraculous by any fair use of the term. These feats have been particularly potent when deployed as weapons against the servants of Chaos, daemonic and mortal alike. It may very well be possible, that in fact, as the ancient credo tells us, the Emperor protects...

Warp Entities
The Warp, through a realm of pure psychic energy that is inimicably hostile to all forms of organic life from realspace, is home to its own forms of life, sentient and otherwise. Many of these creatures are not sentient but depend upon the absorption of psychic energies to survive and are thus naturally drawn to the psychic emanations of mortals in realspace whose psyches are the very wellspring of the Empyrean. Such creatures include the Vampire, Psychneuein, Astral Spectre, Astral Hound, and the hideous Enslavers.

Enslavers are spontaneously created by the shifting energies of the Warp from the nightmares of sentient species, and prowl the Immaterium like sharks. The Enslavers are able to psychically control other species, forcing them to their will, hence their name. Enslavers are able to control almost any species, even creatures such as Tyranids and daemons! However it is thought that the only race immune to the Enslavers are the Necrons, who have no souls nor psychic presence in the Warp. Enslavers exist primarily in the Warp, but can enter the physical universe like a daemon through the unprotected mind of a human psyker. They are able to do this when three Enslavers band together to form a 'trinity', whereupon they form a psychic bond with their psyker victim. Over the course of the next 2-3 days after contact is made, the psyker's organic chemistry changes significantly, the end result being a living, pulsing Warp Gate, through which the Enslavers travel into the material universe. Enslavers are rather like an octopus in shape, their main body being made up of two roughly circular parts, one on top of the other. They have 8-12 small tentacles protruding from their bottom half, and 2 larger tentacles ending in suckers. On the smaller, upper half of their body, they have a sensory organ sometimes referred to as an 'eye'; this circular organ is normally reddish in colour. Their leathery skin is able to change colour, but is normally a light brown with pale tentacles.

But far worse than these creatures are the truly intelligent denizens of the Warp, the Daemons of Chaos. Starships that travel through the Immaterium by use of a Warp-Drive are shielded from attacks by these creatures by their generation of a Gellar Field. Should this field's emitters fail or malfunction, the crew of the ship will be at the mercy of all of these horrors, and will likely be consumed by daemons or become their mortal playthings. The Warp is also the home of the greatest of all terrors, the four major Chaos Gods -- Khorne, Nurgle, Tzeentch and Slaanesh -- who each dwell within a separate kingdom within the Realm of Chaos, the existential embodiments of the darkest aspects of countless sentient species' psyches. Though the Immaterium and the Ruinous Powers are essentially composed of the same kind of psychic energy and the terms "Warp" and "Chaos" are often used interchangeably by those who are aware of the nature of the Immaterium within the Imperium of Man, each Chaos God maintains a primary form that embodies their nature and that is instantly recognisable to their legions of daemonic and mortal followers.

Daemons
The Chaos Gods are not alone in Warpspace. They have created servants from their own essences -- the creatures that mortals have named daemons based on their ancient legends and religious mythologies -- who are not so closely bound to the Warp. Daemons are entities of a somewhat different nature to their masters, and are the most numerous of the creatures to be found in the Empyrean. A daemon is "born" when a Chaos God expends a portion of its own power to create a separate being. This psychic power binds a collection of senses, thoughts and purposes together, creating a personality and consciousness that can move within the Warp. The Chaos God can reclaim the independence it has given to its daemon children at any time, thus ensuring their loyalty. It is only though the loss of this power that a daemon can truly be destroyed, its mind dissolving into the whirls and currents of Warpspace.

Daemons have no physical presence within the Warp. The Realm of Chaos is anathema to the laws of physics and the starships that navigate its depths do so by taking a skin or bubble of "reality" with them when they enter using their Warp-Drive. Instead of possessing a true physical form, daemons project a form conjured from raw psychic energy that is essentially a lesser interpretation of their master's fundamental nature. Hence, the bizarre and inhuman appearances projected by daemons indicate their presence, status and allegiance to a Chaos God.

Though it may appear to be made of normal matter when it materialises in realspace, a daemon's form is no more physical than it is in the Realm of Chaos. In fact, they are beings of pure Warp energy given shape and depth. When manifested in the material universe, daemons have particular invulnerabilities and weaknesses, as well as many strange powers derived from their Warp-born nature as psychic beings. Slaying a daemon's physical projection does not kill it, but only severs its presence in reality; its true essence in the Warp remains unharmed.

When a daemon is "killed" in the material universe, it is banished back to the Warp. If not simply re-absorbed by its creator, it must remain there to regain its strength that it eventually might manifest itself again. Legend has it that a daemon banished in this way cannot return for a thousand Terran years and a day,, though it is of course impossible to prove such a belief through study, and the concept of time itself is meaningless within the Warp. The slight to a "slain" daemon's pride is considerable, however, and the daemon is forced to endure the mockery of its fellows until it can return to corporeal form and avenge itself. The most powerful daemons will call upon any servants and tributary Lesser Daemons to help them achieve their revenge. If it has many allies, it may also request their aid, though all daemons are cautious in doing so. Such favours must inevitably be returned, and no daemon welcomes the dominion of another creature, be it mortal or daemonic.

Daemonic Incursions into Realspace
In order for a daemon to break through into the mortal universe, there must be a breach of the barriers between Warpspace and the material realm -- a Warp rift (also see below). These are breaches in the fabric of reality that can vary in nature and size, such as the massive Eye of Terror or the Maelstrom. Sometimes these occur randomly; at other times, either mortals or the Chaos Gods themselves bring about their creation by some supernatural act. The size of the breach can vary tremendously, from a slight thinning of the dimensional walls to immense wounds in reality that allow the daemonic legions to invade en masse.

At times when certain conditions like the appearance of a Warp Storm or a sorcerous ritual have weakened the barrier between the Warp and realspace, a daemon can possess a mortal and turn him or her into a living portal through which whole daemonic hosts can pass for a time into the material universe. These daemonic incursions can taint realspace severely, often twisting and reshaping whole planets until they are lost into the Warp, becoming a part of the Immaterium and thus transformed into Daemon Worlds. It is not surprising that the Inquisition's Daemonhunters, the Ordo Malleus, are granted unlimited resources and political power by the High Lords of Terra to deal with such terrible threats to the continued existence of Mankind.

Often, it is the tumultuous movements of the Warp itself that create a break into the material realm, allowing daemons to spill through the resulting breach. This might happen by chance; events such as the onset of Warp Storms, Warp-Drive implosions or a rogue psyker's mind suddenly exploding with raw power can cause a rift to appear into the Immaterium. At other times, the deliberate rituals and blood sacrifices of Chaos-worshipping mortals can allow the teeming hordes of the Chaos Gods to smash through into the material realm. Sometimes, simple mortal suffering, death and misery on a massive enough scale can form a Warp rift that a daemonic legion might use as a portal. Some Warp rifts last mere hours, or even moments, for the nature of Chaos is ever impermanent. A daemonic army that has passed through such a rift can become trapped in realspace and will swiftly succumb to the constant leeching of the Chaos energy required to maintain its presence.

Of these, the greatest and most dangerous Warp rift is the Eye of Terror, which has lasted for more than ten thousand standard years. The Eye can be seen as a purple-red bruise upon the firmament from fully half the worlds claimed by the Imperium. It was created by the birth of the Chaos God Slaanesh in the early 30th Millennium and is home to unnumbered Daemon Worlds fought over by daemons and mortals alike, the infamous Traitor Legions of the Chaos Space Marines amongst them. Another Warp rift of excessive magnitude is the Maelstrom, found near the galactic core and a haven for thousands of pirates and Renegade Space Marine Chapters. Other, less widely known Warp rifts exist, such as the Heart of Darkness near to the world of Atilla, the Storm of Judgement that engulfs almost all of the Caradryad Sector, Hell's Gullet, which threatens to swallow the Berillian System whole, and the infamous Screaming Vortex that marks the border between the Calixis Sector and the frontier of the Koronus Expanse in the Halo Stars.

Random Warp rifts are often caused by Warp Storms -- roiling expanses of turbulence within the Realm of Chaos that are echosed in realspace, restricting Warp-dependent travel and communication. If they become focussed or powerful enough, they can pull the fabric of reality taut or even tear it open. Compeletely unpredictable, Warp Storms can be isolated to single planets or expand to encompass whole sectors. One of the worst Warp Storms ever recorded in human history cut Terra itself off from the rest of the galaxy for the entire 5,000-year-long epoch known as the Age of Strife. The unfortunate worlds in the vicinity of such a cataclysmic event can become the playgrounds of daemons until the storm finally expends itself. A few Warp Storms have endured for so long that they can be considered permanent, their self-sustaining energies trapping nearby planets and star systems in a quagmire of roiling Chaos.

Without the Warp, there would be no psykers, no interstellar travel and no interplanetary communication. In fact, the Warp is essential to the survival of humanity. Spacecraft travel through its shifting tides, capable of travelling thousands of light years in a fraction of the time a journey using conventional reaction drives at sub-light speeds would take. By such means, humanity is bound in a single interstellar Imperium, led by the divine Emperor of Mankind.

Through astrotelepathy and the guidance of the psychic mutants known as Navigators, the worlds of the Imperium are able to maintain their fragile bonds. The Emperor's will may be mighty, but His reach is long only because His fleets can travel through Warpspace, the fragile bubbles of reality that protect each warship held up by complex Geller Fields and raw faith.

While Mankind would have a fraction of its current numbers and strength without the Warp, Chaos in its turn would be much diminished without the presence of humanity in the galaxy. The Chaos Gods drink emotions and thoughts, growing bloated with psychic power in the process. Over the millennia, each has fed on an aspect of human nature: rage, desire, corruption and inconstancy. Strengthened and moulded by the collective thoughts and emotions of the inhabitants of reality, the Dark Gods nurture in Mankind those same passions that sustain their very existence. As humanity has spread across the stars, its numbers have grown immeasurably, fuelling the Chaos Gods. So the circle is established, with mortal follies and weaknesses feeding the Dark Gods and those same gods then encouraging Mankind in turn to further follies and weakness. Humanity has long been able to use the power of the Warp -- magicians, seers, witches, mediums, shamans and exorcists have all trapped into its power across the breadth of human history, albeit often unwittingly. Psykers such as these manifest their powers by drawing upon the Warp, siphoning its unnatural energy to hurl blasts of energy, teleport objects, send their thoughts across space and time and perform countless other "miracles." Once, the gift of true psychic power was confined to only a few helpless individuals who usually fell victim to superstitious prejudice in Mankind's distant past. However, the number of psykers is rising in the galactic population of humanity with every passing century. This constitutes a profound evolutionary change for Mankind. Every time a psyker draws upon the Warp, he disturbs its natural flow, creating an eddy that may simply die away or be fed by other movements until it becomes a raging tempest that feeds a Chaos God. Each psyker causes a pinprick of disturbance within the Warp; each can be the seed of a Warp Storm; each can rouse a Chaos Power to unthinkable conquest.

The invasion of a daemonic army is analogous to hell being unleashed upon reality. Free from the physical limitations of a mortal force, a daemonic legion can appear and disappear at will. On occasion, it will mass for a great attack; at other times, individual packs of daemons will hunt across the affected globe, terrorising the populace, randomly enslaving and killing. Those mortals with even the least psychic potential suffer first as the influx of Warp energy releases the latent power of their mind, immolating them with magical fire, turning them into rocky statues, or causing their brains to simply explode. Poltergeist activity and random brusts of pyrokinesis can ruin buildings in an instant. People hear deranged screams and lurid whispering whilst unnatural stenches taint the air.

A daemonic invasion is all but impossible to stop by conventional means, for the very act of warring against daemons feeds the psychic power keeping them in realspace with fear and hatred. Only the closing of the Warp rift can deprive the daemons of their power. Often, there is nothing that can be done but battle against the incursion until the Warp rift runs its course. Battles fought against a daemonic incursion are utterly different than those against mortal foes, for defensive structures and garrisons have little to no effect. To wage war against invading daemonic hosts, an army must be ready to respond to the most sudden appearances of its adversaries.

To compound matters, the motivations behind daemonic incursions are unfathomable. Even the Tyranids, who are so alien as to be beyond the ken of man, war for sustenance and survival. Instead of these base drives, the objectives of a daemon commander will often be completely obscure -- they might be to slay a million mortals, to retrieve a single artefact, or to kill the grandchildren of those that once banished them back to the Warp. Other times, the Daemons have no goal or plan at all, and the gibbering creatures of the Immaterium make decisions according to opportunity and their intrinsic nature.

To fight a daemon army is to fight a twisting tornado of unreason and despair. Even then, the Imperium cannot allow the knowledge that such foes actually exist to spread. The human survivors of such conflicts are invariably confronted by the Inquisition and mind-wiped, quarantined in forced labour camps or even -- in extreme cases -- become the subjects of a worldwide Exterminatus event. Over the aeons, the galaxy has witness Warp-based catastrophes and daemonic incursions beyond counting. Since the inception of the Imperial Inquisition in the early days of the Horus Heresy, even the fact that such a thing is possible is deemed too dangerous for the citizens of the Imperium to know, for such knowledge breeds heresy as surely as a flyblown corpse breeds maggots. because of this, the vast majority of knowledge concerning daemonic incursion has been eradicated from extant Imperial public records. What is known is recorded only in proscribed Imperial texts and heretical xenos scripts that the Inquisition has yet to destroy.

Warp Rifts
On rare occasions, the realm of the Immaterium is able to break out into the physical universe in what is known as a Warp Rift. A Warp Rift is a weakening of the barrier between realspace and the Empyrean, allowing its creatures to enter the material universe. Sometimes, Warp Rifts occur randomly, while other times, the Chaos Gods or mortals manufacture them. Some last mere moments, others for days, years, or even for Terran centuries. They can take many forms, often a seething whirlpool of raw psychic power, other times as a terrifying glimpse into the Realm of Chaos. They have also appeared as gates of green fire, clouds of impenetrable shadow, boiling lakes of pus, and other terrifying visions capable of driving mortals mad. These Warp Rifts are usually settled by the once-mortal followers of the Ruinous Powers of Chaos, and are often the site of titanic atrocities previously unleashed in the local region of space by these Forces of Chaos.

A number of different factors can induce the formation of a Warp Rift in realspace, including the unpredictable movements of the Warp causing a break into realspace, the daemonic possession of a mortal, deliberate rituals of daemon summoning carried out by Chaos Cultists, a breach in the barrier between realspace and the Warp caused by the psychic focus of immense suffering and death following a great war or terrible atrocity, a Warp-Drive implosion, or as a result of the unified will of the major Chaos Gods.

The most well-known Warp Rifts in the Milky Way Galaxy include:
 * The Eye of Terror, created when the Eldar inadvertently caused the creation of Slaanesh, and is now home to the Traitor Legions of the Chaos Space Marines and their other human followers like Chaos Cultists, Traitor Imperial Guardsmen and the Hereteks and Traitor Titan Legions of the Dark Mechanicus.
 * The Maelstrom
 * The Storm of the Emperor's Wrath (currently consuming much of the Caradryad Sector)
 * Van Grothe's Rapidity (near the world of Medusa V)
 * Heart of Darkness (near the Attila System)
 * Hell's Gullet (in the Berillian System)
 * The Screaming Vortex (between the Calixis Sector and the Koronus Expanse)

Warp Storms
The Warp is an extremely volatile medium. At times, disturbances can turn areas of the Warp into raging storms of incomprehensibly destructive fury that can breach the barrier into realspace. These storms can last for days, months, or even standard centuries. These "Warp Storms" can isolate star systems and entire sectors from each other, by making the Imperium's normal methods of interstellar travel and communication impossible as the Warp surrounding those regions becomes too chaotic for a starship to safely travel through or for an Astropath to send or receive a telepathic message through the psychic "interference." These storms can last for any amount of time, ranging from weeks to centuries, and can occasionally break out into the material universe, creating a temporary Warp Rift that can result in an incursion by the Forces of Chaos into Imperial space. Starships caught in these storms are likely at best to be driven far off-course and emerge back into realspace thousands of light-years from their intended destination in uncharted areas of the galaxy. More often, such vessels find themselves trapped within the Warp, a terrible fate for its passengers as they become playthings for the dark creatures that inhabit that malign realm when their protective Gellar Field finally fails. While uncommon, some starships, after making their way through the Empyrean for only a short time, will emerge from the Warp only to find that entire Terran centuries have passed in realspace. Such an occurrence is more likely to happen when a vessel is travelling through a region of the Warp affected by a Warp Storm or unusual psychic turbulence. However, the Warp is a dangerous and chaotic place at the best of times; every use of a Warp-Drive is perilous, no matter how routine.

Controlling the Warp
There are conflicting accounts as to whether it is Tzeentch, the Emperor of Mankind, or a creature known as the "numan" (from the Inquisition War Trilogy) that prevents the Immaterium from spilling out and completely engulfing the material universe. However, recent canon seems to imply that the anti-psychic technology of the Necrons contributes, too. It is also possible that the laws of physics act as a barrier. It should also be noted that the Immaterium seems to only be relevant to the Milky Way Galaxy (although the Tyranids' case poses a question), implying that it is either a unique occurrence, that there may be other "Warp shadows" of other galaxies where sentient life exists or that the Warp continues in other areas, but uninhabited by daemons due to the lack of sentience. A paradox of thoughts is apparent: the extra-dimensional entities of the Warp are sustained by the minds and souls of living creatures within the physical world. So what would occur to the Warp if all life was obliterated through the fall of the physical reality? It would probably cease to exist.