An Astartes chapter, now erased from history once called this deathworld home. They discovered an ancient, subterranean xenos portal of some sort and either tried to open or destroy it. Instead the portal fractured, tearing a hole in the fabric of reality that could not be closed. Despite their desperate attempts to contain the disaster, they were lost in the ensuing cataclysm.
The Inquisition stepped in and ordered Exterminatus, but not even their cyclonic torpedoes could close the portal. Nor eliminate the wretched taint of the warp emanating from it. Prism was erased from all maps and star charts, yet it's screams in the ether mark it as a blazing beacon of terror to Psykers and Navigators alike.
Periodically, the High Lords will dispatch an Astartes chapter to the world to cleanse it of whatever foul horrors have populated it via the warp breach, be they Xenos, Daemon or Heretic. Every arriving strike cruiser announces itself with an orbital bombardment, but it is just a formality. Only boots on the soiled ground, bolters, blades and holy flames can properly cleanse this blighted planet. For a little while, at least.
The last chapter to take up this grim duty was the 5th Battle Company of the Black Dragons, the proverbial foxes guarding the hen house. Only the Emperor knows what trophies those vile mutants collected. Rather than purging this rock of the warp's taint the Dragons likely saw it as an opportunity for a recruitment drive. It is only the sad state of the Imperium of Man that such tainted Astartes are even tolerated.
