These ring-like structures are made of Blackstone and have the ability to summon the powers of the Warp. Over the course of the Gothic War, Abaddon learned that 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 searing visions. All of this serves to bring the forces of the Warp to the planet, strengthening psychic power but also making it more easy for the forces of Chaos to bring Daemonic allies to Realspace.
Channeling 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 supplicant that sought to harness its power.