The Collar of Khorne that hangs around the model's neck is said to be forged from the heat of Khorne's rage at the very foot of the Blood God's throne of brass. The collar is able to suck the energy of the warp from around it, fortifying the wearer and also protecting it from psychic attack. As a result, force weapons lose their special ability to kill the wearer outright, and psychic powers that target the wearer or include him in their area of effect are nullified and will not work on a D6 roll of 2+.
Collar of Khorne
Source: Codex: Chaos Space Marines (1999), p. 28