Over the course and many retcons of the Gothic War, Failbaddon learned that, after fucking around with the material, Blackstone can not only repel the Warp but also attract it. Their construction is overseen by Masters of Possession, who plant the structures at sites of geomantic and ritual significance.
Wherever Noctilith Crowns are planted, the minds of Chaos Psykers are flared with a frisson of forbidden power. Those who have any form of psychic sensitivity find strange new phenomena manifesting around them. Even slaves and Cultists without a flicker of psychic potential can be assailed by super heretical visions. All of this serves to bring the forces of the Warp to the planet, strengthening psychic power but also making it easier for the forces of Chaos to bring Daemonic allies to Realspace.
Channelling the power of a Noctilith Crown is a risky affair due to the potential of tremendous psychic backlash. More power than any mortal can handle will flood into the minds of those supplicants that sought to harness its power.
The spiky Stargate wannabe grants all Chaos units a 5++ and a re-roll to failed psychic tests as long as they stay within 6", and the aura expands to 9" and 12" on the second and third turn of the battle. It also debuffs non-chaos psykers by forcing all doubles rolls to act as Perils within 24" and it can defend itself with a 6" Pistol d6 S7 AP-2 2D shooting attack at BS 4+.