N'Kari
Well, N'Kari is a Keeper of Secrets, not a Daemon Prince, that is older fluff.
She/He (Presumably He, but I guess Keepers of Secrets are Hermaphrodite, so it doesn't matter that much) Did some funky stuff, and was at Horus' Battle Barge at the final moments of the Siege of Terra, where he was banished by no other then the All-Mighty Corpse Emperor, and sent back to Slaanesh to have a rape party afterwards.
He also was doing some scavenging on a derelict Craftworld, called Malan'tai, where he was "Om Nom'ing" the Eldar Spirit Stones, until some of those retarded SUPER KNIGHTS OF THE EMPEROR Swooped in, kicked he's demonic servants, then got their asses kicked by N'Kari until something random happened, and he was defeated.
There isn't that much fluff behind him, but we can all assume He's generally badass, or not that badass, like any other Keeper of Secrets, but as they are Greater Daemons of Slaanesh, they're badass, but not as badass as Greater Daemons of other Chaos Gods
He also made an appear in Warhammer: Online, but no one cares about it. He's also not even close to being as badass as Doombreed, which we can all agree on.