The Companions of Frost
A Greater Daemon of Tzeentch
Was connected to Arch Cardinal Ignato as one of the orchestrators behind his rise to prominence within the Ecclesiarchy and his membership within the heretic cult: the Maledictors Hand.
The heresy of the cardinal was uncovered by members of the Zanatov Rogue Trader Dynasty under Inquisitorial sanction, who gathered evidence against the cardinal and managed to bind the daemon to the core of a lost Imperial Titan at the cost of their own lives and leaving the mystery forgotten.
It was only after The Acolytes uncovered a piece of the Dei Phage which remained in the material realm that led their investigations to the missing titan and therefore inadvertently un-bind the daemon, allowing it loose.
All was not lost, as the recovery of the Titan also uncovered the missing evidence against the Cardinal and allowed those investigations by the Zanatov dynasty to continue and follow those leads to investigate Arch Cardinal Ignato himself.
CHURCH OF THE DAMNED
Things were not as they seemed however, as Ignato’s companions within the Maledictors Hand had considered him to be too unstable and power crazed for their purposes and had long since had him replaced by an infiltrator. (Presumably when he lost his connection with the Dei Phage after its conflict with the Zanatov Dynasty)
The real Cardinal was held in stasis until which point his captor was executed. Newly freed; Ignato was seemingly possessed of impressive warp powers and escaped to Stonegarden: The final resting place of Saint Drusus.
The Acolytes destroyed Ignato too late and he had already summoned the recently freed Dei Phage back into the material realm, this time within the body of Drusus himself.