Collars of Khorne are among the many gifts granted by the Blood God to his greatest Champions. Hence, many of his minions, including Daemons and Chaos Beasts, wear them as well, all to better spread the destruction of their vicious master.
These foul collars are studded with spikes on the outside and inside. Made from interlinked pieces of iron and brass, the spikes are always covered in old blood and bits of skin and hair. When worn, the spikes burrow into the flesh, infusing the wearer with the protection of Khorne, giving the wearer a strong resistance to spells. Indeed, weak-willed spellcasters that target the wearer often find their magic snuffed out.
Once donned, however, the Collar may never be removed. If worn by someone other than whom the Collar was bestowed upon, that individual suffers mutation, or worse, becomes a Chaos Spawn.
Source
- Warhammer Fantasy RPG 2nd ED -- Tome of Corruption (pg. 181).