Signus Prime

Signus Prime was the primary 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, though the price was high in both casualties and 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, and its few surviving inhabitants were resettled in other Imperial star systems.

History
Once a major industrial region of the Ultima Segmentum, in the days just before the Signus Campaign, the Signus Clusterfell prey to the agents of Chaos working on behalf of the corrupted Imperial Warmaster Horus as part of his stategic planning for his rebellion against the Emperor. Within a relatively short period of time the entire cluster was transformed into a veritable extension of the Realm of Chaos in the Warp, an entire star system of Daemon Worlds ruled by the Greater Daemon of Slaanesh called Kyriss the Perverse.

Once the system was under the control of Chaos, Kyriss raised for itself a great palace on Signus Prime that he named the Cathedral of the Mark. Then the daemonic legions under his control swept from Signus Prime across the rest of the system, destroying millennia 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.

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 the system 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 recrods 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.