Doomrider
His name's Doomrider. His head's on fire. He does cocaine. I'm afraid that's all we know.
Doomrider was an old Slaanesh special character that had to be summoned, and would disappear if the player rolled a one on a special die roll at the beginning of his turn. This could lead to him disappearing on the same turn he was summoned, completely wasting the points spent on him.