The term Zone Mortalis -- the "fatal ground" in High Gothic -- was the term used by Imperial military forces of the Great Crusade and Horus Heresy eras to refer to the most savage arenas of combat conceivable. The concept of the "Zone Mortalis" as a theatre of warfare dates to the era of the Great Crusade as referred to in the ancient Codex Tactica Imperialis. Such zones, be they the contested decks of a void warship, wall breaches, tangled mine works, lightless under-hives, the prison-vaults of sundered fortress citadels, labyrinthine industrial sewer systems and sacred catacombs, all have a confluence of factors in common such as close confinement, limited access for attack or escape routes, as well as treacherous environments, which make them murderous venues for warfare. It was an area in which the brute force of the Legiones Astartes excelled, and much application of strategy evolved from this most deadly form of warfare. Zone Mortalis battles are often fatal and chaotic affairs, such as fighting in a collapsing hive city under mass artillery bombardment or on a burning star vessel in the middle of a pitched void battle. Some from Legions such as the Ultramarines and Iron Warriors saw victory in the Zone Mortalis as a precise application of tactics and equipment, while others such as the Blood Angels and World Eaters relied on courage and fury to carry the day, but without doubt the undisputed masters of the Zone Mortalis were the Death Guard Legion. In such warfare they were relentless, merciless and without peer.
Zone Mortalis Missions
Boarding actions between starships are truly deadly affairs, vicious and swift close-quarter duels in the most treacherous and dangerous environments imaginable. In short, the kind of conflict the Space Marines were created for. The vessels of the early 31st Millennium ranged in size and role from interplanetary cargo scows to mighty Battleships with crews numbering in the thousands serving as the flagships of the Imperium's far-ranging Expeditionary Fleets. Larger still were Mechanicum mass-conveyers tens of kilometres in length, and unfathomably ancient drifting Space Hulks that dwarfed even these. In the case of these larger vessels, regardless of whether they were an engine of war or a colony ark, their sheer size granted them weaknesses to be exploited, just as their immense bulk made them difficult to destroy by more conventional means. For these mighty voidships, damage to specific systems could be as fatal to them as poison injected into a living heart. The best way of doing this was often by breaching the hull of an enemy vessel and assaulting it with troops -- if they were victorious then the ship might be crippled, captured intact or even destroyed from within. Only the most skilled and able warriors were selected to undertake boarding assaults and attrition among them was always high, as was the glory gained should they succeed. The shipboard defenders who repelled a boarding assault had to fight with all the courage and ferocity they could muster if they were to save their vessel. For the loser death was certain, as aboard a starship surrounded by the infinite darkness of the cold and silent void, there was nowhere to retreat or run to.
Most boarding actions take place against the backdrop of a fierce void battle in which immensely destructive forces quite beyond the combatants' control are unleashed that may even destroy the very ship through which they are fighting. Even when the boarded ship is not taking direct hits in the battle, the effects of prior damage can cause secondary explosions to rip through its hull or the vessel itself may be caught in a crossfire or ride through the blast waves of dying ships and lethally spinning debris. A "hit-and-run" attack is a crucial tactic used in ship-to-ship combat and involves one or more boarding parties forcing access to the target ship, fighting their way through the companionways with the aim of destroying key systems. Boarding an enemy vessel with the aim of destroying, crippling or capturing it is a gruelling task and one that will exact a high price in blood from both attacker and defender over a number of savage battles.
A void strike mission represents the most perilous phase of a boarding assault for an attacker -- the attempt to establish a foothold on an enemy vessel from which to press forward towards its most vital areas. The interior of a starship is a tangled labyrinth of passageways and chambers. Wrecking missions represent an attack on one of several vital target areas, ranging from the warship's massive Plasma Drives to its command deck. The larger the starship, the more of these systems that attackers have to cripple in order to bring about the death of the vessel. Sometimes an attacking force will utilise a desperate gamble, and instead of launching a boarding mission against the target vessel's more easily accessible launch bays or loading decks, they attack the outer, armoured skin in an attempt to cut through and strike directly into the vessel's undefended and most vulnerable systems. A successful killing blow can conclude a hit-and-run attack before the defenders have even mustered to repel the boarders. If the attackers fail, they are doomed to a cold death as the void claims them.
The final stage of a boarding assault mission is when the attackers have to make their desperate withdrawal from the target vessel. It may be that the boarders have succeeded in their mission to cripple the enemy warship and must now withdraw to their assault boats or teleportation points even as the starship tears itself apart around them. Conversely, it is entirely possible that the boarding action has met with failure having been repelled or perhaps recalled by the parent vessel, necessitating a hasty retreat as the voidship's vengeful crew close in from every quarter.
Encounter missions are a special Zone Mortalis mission where two hostile forces advanced through unknown ground where neither side had the advantage of foreknowledge or tactical control of the area.
Search and Destroy
This mission represents the vicious struggle to control the Zone Mortalis through brute savagery and attrition, destroying the enemy's forces in detail.
This was when two opposing forces fight to control the area of the Zone Mortalis and drive out the enemy.
This was a mission in which two opposing forces seek to fight their way into enemy-held territory, slaughtering as many of the foe as possible.
Common Dangers of Zone Mortalis Operations
- Deadly Ground- Fighting within Zone Mortalis terrain is a dangerous and treacherous affair, and one in which even light vehicles can flounder or damage themselves, unexpectedly crashing through buckled deck plating, getting crushed beneath falling rubble, or contacting exposed power conduits with deadly results.
- Firestorm and Shrapnel - The confined spaces of the Zone Mortalis can prove to be death traps to the unwary and certain types of weapons have their effectiveness increased by the environment, while others become more unpredictable.
- Nowhere to Hide - Units that break and flee from combat within the confines of a Zone Mortalis are far more likely to be trapped in areas where their avenues of retreat are entirely cut off, and due to the confusion of corridors and passageways that may surround them, falling back may be a particularly deadly affair.
- Reaction Fire - The confined spaces of a Zone Mortalis make for deadly, close-range fire-fights, where a split second reaction may be enough to gun down an enemy rushing out from the darkness before death strikes a Space Marine down.
- Catastrophic Damage - One of the perils of battle underground or within a building in a warzone is the risk of bringing the roof down on themselves and being buried alive or crushed under tonnes of earth or masonry.
- Enemy Unknown - Fighting in levels of low visibility over distance, is nothing compared to the abyssal darkness of fighting deep underground or within the tortuous confines of a Space Hulk, where even the finest auguries and sensors may prove utterly useless.
- Outer Hulls - The expanse of a gigantic starship's outer hull can itself form a battlefield, and some of the most desperate and bloody boarding actions are fought simply to force a breach in a voidship's armoured flanks. In boarding actions of this kind, the outer surface of a voidship forms the battlefield for attacking forces, with them seeking either to force air locks open or damage vital structures on the hull's surface.
- Cold Void and Poisoned Air - Fighting in a Zone Mortalis aboard a vessel has many dangers in its own right, such as areas filled with poisonous gas, choking industrial fumes or extreme heat, as well as the effects of fighting in a depressurised area of a space vessel during a boarding action.
Zone Mortalis Stratagems
Zone Mortalis stratagems was when forces would utilise unique and unusual tactics, taking advantage of the terrain and equipment available to a Space Marine Legion force which would play an integral part in the successful completion of their mission. The use of such stratagems was neither compulsory nor, strictly speaking needed, but they could add a new dimension and a few nasty surprises to both attacking and defending forces on both sides.
- Tunnel Access - This is when forces gained access to the conduits and service tunnels in the area of the Zone Mortalis, providing a Space Marine commander with a significant advantage -- just as long as they didn't collapse first.
- Breacher Charges - Some Space Marines were equipped with a single breacher charge, a combat explosive device designed for breaking into bunkers and blasting apart bulkheads.
- Flanking Counter Assault - This particular stratagem was when a Space Marine commander has sent a portion of their forces off in an attempt to outflank the enemy in the hopes of cutting them off, enabling them to be isolated and destroyed.
- Lascutter (Gear) - Some Space Marines were equipped with lascutters, a powerful industrial tool that could be used for cutting through armoured bulkheads or as a makeshift but devastating close-quarter weapon.
- Spearhead Sentry Gun - Both attacking Zone Mortalis forces and defenders often deployed automated weapon systems to provide them with fire support, usually covering a vital area or important access chamber. This weapon was a light artillery mount equivalent to an Imperial Tarantula, heavy Gun Servitor or defence platform.
- Interdiction Assault - Either through the use of advanced phase-field generators to render the surrounding terrain temporarily out of sync with reality, or the rather more brute force approach of blasting out intervening ferrocrete and earth with seismic charges, the attacker could make a major breach in either the ceiling, floor or walls from which they would have troops poised to make a storm assault.
- Sustained Assault - When attacking forces greatly outnumbered those of defending forces, this allowed them to press on heedless of casualties with reinforcements close at hand.
- Fire Wasp - A Fire Wasp was a Mechanicum-produced combat drone used since the early years of the Great Crusade in Zone Mortalis actions to venture ahead of assault parties and help clear them a path. Sent in ahead of a squad and intended to trip mines and other booby traps, the Fire Wasp was an extremely robust device, if a poor fighter. Most standard patterns were equipped with a built-in Flamer weapon and Searchlight device for aid in their mission.
- Defensive Strongpoint - After deployment areas were worked out but before any Space Marines were deployed, the defending forces would often times utilise an enclosed or semi-enclosed area of terrain to be their defensive strongpoint.
- Barricades - This was when defending forces has had enough time to get ahead of their attackers and meet them on prepared ground, utilising barricades and other barriers as protected positions and killing zones.
- Traps - The defending force has been able to seed the area with anti-personnel mines, tripwire-bombs, rad-fields and hidden deadfalls, making it a potential death-trap for attacking forces.
Zone Mortalis Boarding Assault Stratagems
- Void Hardened Armour - A single unit in an boarding assault strike force is equipped with specialised additional armour and life support systems.
- Shock and Awe - The assault force calls in a thunderous barrage of ship-to-ship fire, cluster grenades or even teleporter charges in an attempt to disorient and throw the defenders off-guard. However, such attacks are seldom as precise as the attacker would like.
- Phase Field Generator - A single Space Marine is equipped with a back pack-mounted Phase Field Generator which allows the individual and his squad to phase through a closed bulkhead door or through a single Zone Mortalis wall section. This is a very dangerous proposition as a phase field could flicker out of sync and trap a warrior within the stuff of the bulkhead, and slay his fellow squad members outright.
- Devastation Drone - Devastation drones are mobile demolition bombs armed with implosion charges designed to inflict maximum damage within a highly confined area (and thus reduce the risk to the voidship they are used on). They are compact, man-sized and quite sturdily constructed in order to resist enemy fire until they have reached their target zone. Many are often mounted on heavy track units to keep them mobile, although the best have anti-grav plates instead.
- Security Airlocks and Bulkhead Doors - Vital access points controlled by defending forces during a boarding assault action.
- Murder Servitor Auto-Clade - The warship is equipped with chambers within which are sealed small numbers of skull-faced Murder Servitors, their hands replaced with grafted-on billhooks, spikes, axes and other equally crude but effective weapons. At the captain's command, every chamber is unsealed and the Murder Servitors are activated, tearing forth in a savage tide to scour the ship of boarders.
- Voidsmen Gangs - The defender is forced to press the voidship's crew into the defence of the vessel. Equipped with armoured void suits and emergency weapons, these crewmen are often far from the best-trained soldiers, but will be well-motivated, as nothing but death or slavery awaits them should their vessel be taken.
- The Horus Heresy - Book One: Betrayal by Alan Bligh, pp. 168-181