Warhammer Fantasy

A Chosen of Chaos is a warrior marked out by one of the four chaos gods for doing something they found impressive or interesting and is in the process of either becoming a Chaos Lord or a, er, That-Which-Must-Not-Be-Named if they fail or can't handle the "gifts" bestowed upon them.

A Chosen of Khorne has gained the patronage of his god by doing Khornate things, such as fighting honourably and bullying Tzeentchian nerds.

May also have done something involving blood and/or skulls as well, possibly.