Well I don't know about the recent changes but he can't have changed that much since 2nd edition
Anyway we all know that Harlequins are a trope of entertainers who perform acts of eldar history, stories etc. amongst many other things
Each troupe has only 1 solitaire who plays the role of Slaanesh. Although part of the troupe, he lives by himself and stays away from the rest. He is also only ever really spoken to in ritual form. Because he plays the part of Slaanesh, he must follow the path of damnation which means he doesn't get a spirit stone and that his soul must be fought over between the laughing god and Slaanesh; the laughing god can only win if he somehow tricks Slaanesh, something which rarely happens.
Due to all this, he is the most dangerous of all Harlequins; to cross his path is to invite damnation, and that if you accidently touched or crossed him, you might as well kill yourself, as you most likely to die a horrible death otherwise.
On a side note, they have never been allowed to lead squads of troops, the name somewhat gives it away.