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)
According to latest studies, N'Kari once was a human, native of Old Earth, an actress of dubious morale, going by name of Sasha Grey. Such was the power of lustful pleasure associated with her that eventually the warp reflection of her soul transformed into a sentient being of unimaginable passion and lust. After the realization of her own existence, she instantly sensed the perturbations in Warp, which was the formation of Slaanesh and immediately recognised it as her future master. N'Kari then patiently waited for the birth of new Chaos God and totally and undoubtedly give herself to him.
She did some funky stuff after that and also, was at Horus' Battle Barge at the final moments of the Siege of Terra, where she was banished by no other then the All-Mighty Corpse Emperor, and was sent back to Slaanesh to have a rape party afterwards.
Also fought the Phoenix Lord Asurmen during The Fall killing him for the first time because his backup, Drastanta Phoenix Lord of the Shining Spears was out getting smokes, but Drastanta showed up and blew N'kari with his lance.
She also was doing some scavenging on a derelict Craftworld, called Malan'tai, where she was "Om Nom'ing" the Eldar Spirit Stones, until some of those retarded SUPER KNIGHTS OF THE EMPEROR Swooped in, kicked her demonic servants, then got their asses kicked by N'Kari until something random happened, and she was defeated.
There isn't that much fluff behind her, but we can all assume she'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. any truly interested person surely can find some really cool videos of her human times.
She also made an appearance 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.