Signus Prime, officially Signus III, was the third and capital world of the Signus Cluster, a densely populated trinary star system comprised of seven Hive Worlds and fifteen inhabited, heavily industrialised moons. During the Horus Heresy, Signus Prime was the primary battlefield of the Signus Campaign fought by the Blood Angels Legion in 005-006.M31.
Ordered by the Warmaster Horus to cleanse the Signus Cluster of the xenos species known as the Nephilim who had supposedly conquered the triple system and subjugated its human population, in fact, the Signus Cluster had been overwhelmed by the daemonic forces of Chaos. Horus hoped that the Blood Angels and their Primarch Sanguinius would be either destroyed by the Chaos forces or corrupted by them to his side. Either way, a powerful Loyalist Space Marine Legion would no longer be able to interfere with the Warmaster's plans to betray his father, the Emperor of Mankind, and launch an offensive against Terra to overthrow His rule.
Ultimately, neither Sanguinius nor his Blood Angels were corrupted by Chaos, and they managed to defeat all the daemonic forces thrown against them. But the price was high in both casualties and due to the first onset of the genetic Flaw later known as the Black Rage among the Astartes of the IX Legion. In the wake of the Signus Campaign, the Blood Angels became determined to return to Terra and defeat the Traitors led by Horus who would turn to the service of entities as vile as those they had faced on Signus Prime.
The Signus Cluster itself was quarantined from all further contact with the Imperium by warning beacons, and its few surviving, uncorrupted inhabitants were resettled in other Imperial star systems.
For untold generations, the Signus Cluster had been a centre for human civilisation. Great hive cities littered the seven worlds of the Cluster and huge industrial complexes burrowed into the fifteen moons. But amongst the uncounted millions settled within the Signus Cluster lurked sinister Chaos Cultists.
After his own corruption, Horus had set many plans into motion to prepare for his rebellion against the Emperor of Mankind. One of these had been to send agents to Signus Prime, the capital world of the Signus Cluster, from Davin and other worlds with a strong Chaos presence. These agents infested the rabble of the Signus Cluster and in their teeming numbers hid the witch-cabals that later summoned daemons to Signus. When these agents and Chaos Cultists received a signal from Horus not long before the Istvaan III Atrocity in 005.M31, they rose up in a violent insurrection and seized power within the Signus Cluster, summoning daemons into realspace to lead them and rule over the star system.
Daemons soon stalked the hive cities and established foul palaces across the entire star system. The human population of the Signus Cluster was enslaved, reduced to fodder for the minions of Chaos. Powerful summoning spells cast by the witch-cabals allowed the daemons free reign over Signus. In addition to this, thousands of Signan psykers had been possessed by Warp entities and become Daemonhosts.
The Signus Cluster had become a veritable extension of the Realm of Chaos within the Warp in only a short time, and a demesne of daemons. Kyriss the Perverse, a Keeper of Secrets and one of the Greater Daemons of the Pleasure God Slaanesh, now ruled the Signus Cluster. Once the cluster's primary world was under the control of Chaos, Kyriss raised for itself a great palace on Signus Prime that it named the Cathedral of the Mark. This blasphenous edifice was crafted from the bones of every man, woman and child murdered by the servants of Chaos on Signus Prime.
Then the daemonic legions under Kyriss' control swept from Signus Prime across the rest of the system, destroying the work of millenniums of human civilisation and eradicating the populations of whole worlds. Almost the entire population of the Signus Cluster was enslaved, corrupted to the service of Chaos or just butchered outright, and hope fled from those few who survived.
The 7 worlds and 15 moons of the trinary system were soon in thrall to this mighty daemon lord and its teeming hordes. The daemons were aided by the machinations of the Chaos Cultists. These bands of madmen opened their worlds to the creatures of the Warp, conjuring powerful summoning spells and offering themselves up for possession.
The rest of the human population of Signus had been annihilated or enslaved, their only fate to be consumed by the horrors of the Warp. The entire star system became a charnel house, a place of evil and a home for the creatures of the Immaterium. There was never so base a betrayal as this one, for Sanguinius looked to Horus as a brother, and never would have believed he, his Legion and literally billions of people would be sacrificed to the foul denizens of the Warp.
Their deliverance came in the form of the Primarch Sanguinius and his entire Blood Angels Legion, who came to the Signus Cluster on a mission from the Warmaster Horus to supposedly cleanse it of domination by a xenos species known as the Nephilim. In truth, Horus had sent the Blood Angels to the daemon-infested star system to see them either destroyed or corrupted to his cause.
In the wake of the Blood Angels' unexpected but difficult victory over both Kyriss and the forces of the Bloodthirster Ka'Bandha in the resulting Signus Campaign, the cluster was cleared of all daemonic infestation. A tactical Cyclonic warhead was deployed to destroy the blasphemous Cathedral of the Mark and much of the continent that surrounded it.
Signus Prime and the entire Signus Cluster were thereafter quarantined from the rest of the Imperium and declared mortae perpetua ("forever dead"), forever off-limits to any human interaction through the use of special warning beacons placed all around the system's circumference. The Blood Angels also expunged all of their own records of their presence on Signus Prime to prevent the spread of Chaos corruption, and resettled what few uncorrupted survivors they had encountered on new homes in neighbouring star systems.
- Horus Heresy: Collected Visions (Background Book), pp. 171-181
- Fear to Tread (Novel) by James Swallow, Ch. 20
- The Horus Heresy Book Eight - Malevolence (Forge World Series) by Neil Wylie and Anuj Malhotra, pp. 42-45