Keeper of Secrets

If you thought the Daemonettes were sexually perverse, you haven't seen a Keeper of Secrets, the Great Daemon of Slaanesh. Keepers of Secrets have generally the same body, with four arms and the lower pair ending in pincer claws. What makes them unique is that the head is based on Slaanesh's mood at the time he/she formed the Keeper. As such, it can be anywhere from hauntingly beautiful to insanely grotesque. Like the rest of Slaanesh's followers, Keepers of Secrets seek sensation in all forms, either sex, drugs, rock and roll, or just killing everything around them in a wild frenzy.

On the tabletop, the Keeper of Secrets is a fast CC killing machine. Compared with the Bloodthirster, it's more focused on killing rank and file troops due to it's lacking of the Bloodthirter's beefiness, and it's not as survivable as it due to not being a Flying Monstrous Creature.

Known Keepers of Secrets

N'kari: A Keeper of Secrets (originally said to be a Daemon Prince) ~~responsible for~~ who took credit for the destruction of Craftworld Malan'tai before getting his ass handed to him by the Grey Knights. Also a Elf who was so vain, she wanted the attention of Slaanesh himself and in modern times has been killed by Teclis and Tyrion in the plural.
Zarakynel: Responsible for corrupting the planet Sherilax and one of Slaanesh's most powerful Keeper of Secrets. Enjoys eating the occasional soulstone and Eldar rape.
Daemon of Laeran: A Keeper of Secrets responsible for the corruption of the Emperor's Children. It was found bound to a sword found on Laeran, and had corrupted the Laer into the embrace of Slaanesh. Fulgrim took it as a trophy, and the Daemon corrupted the Primarch. When Fulgrim realized how far he had fallen after killing Ferrus Manus, the Daemon tricked him into lowering his mental barriers and possessed Fulgrim, subsequently impersonating him and committing all of Fulgrim's following atrocities, including the mass debaucheries during the siege of Terra and the seemingly permanent incapacitation of Roboute Guilliman...wait, does anyone else hear clapping? Retcon time! A Horus Heresy novella tells us that after a while, Fulgrim was able to overtake it and trap it in a portrait of himself painted from shit and spinal fluid. Ferrus was Fulgrim's last link to any sort of morality and barrier from a life given over entirely to Slaanesh, so instead of breaking his spirit (which it did for a while), the Daemon of Laeran just set the groundwork for its own dicking over. In the end, the swell and cultured Warlord Fulgrim became a planet rapist and sense freak sadist without a pussified daemon pulling the strings...That clapping is even louder now.
Heartslayer: A Keeper of Secrets who was largely responsible for the downfall of the Eldar Craftworld Kher-Ys and possessing an Avatar of Khaine. Kher-Ys was protected by a particularly powerful set of runes that prevented an invasion. To pave the way for an invasion: Heartslayer seduced the daughter of one of the Farseers of Kher-Ys, had her deactivate the runes protecting her craftworld, and basically made the entirety of the craftworld crap their mesh-armored pants by commencing a full-scale daemonic invasion. The Craftworld tried to summon an Avatar of Khaine, but Heartslayer managed to reach the Avatar's chamber just before the birth of the avatar. For some reason, an avatar has no soul at the moment of its conception, so he/she gouged the avatar's eyes out and proceeded to posses the husk and turn it against the craftworld's defenses. Needless to say, genocidal hilarity ensued.
Kyriss: A Keeper of Secrets responsible for enslaving the Signus System during the Horus Heresy. Committed all manner of unspeakable perversions before Sanguinius and the Blood Angels arrived to fuck him up, only to in turn be attacked by Ka'bandha the Bloodthirster. Horus and Ka'Bandha intended Kyriss to be sacrificed as part of their plan to kill Sanguinius and turn the Blood Angels to Khorne. When Ka'Bandha was thrown back into the Warp, Kyriss told Sanguinius that it would spare the Blood Angels if Sanguinius took the hate of one of his lost sons into him, but the Apothecary Meros did instead. Kyriss proceeded to go absolutely apeshit on the Blood Angels in rage before Sanguinius cut its head off. Interestingly, even Kyriss couldn't seem to decide which sex it was.
The Sapphire King: The physical manifestation of the pent-up grief and pride of the Iron Hands or something like that. I don't know, I was having trouble following exactly why this guy is so fascinated with the sons of Ferrus. Well, besides the fact that he was born when Fulgrim went all chop-chop on Ferrus' neck. Point is, he has a fetish for machines made of fleshy bits and turning Iron Hands into Chaos Spawns so he can-oh shit OH SHIHJKGASDFKGHBGELJWBKC
Emeli Duboir: The Chaos cultist than a certain Hero of the Imperium banged and subsequently killed. Later returned as a keeper of secrets hell bent on sucking the soul of the brave commisar and the planet he was on strait into the warp.
Luxscious: A Keeper of Secrets who was the first to decide he/she/it was Slaanesh's replacement, called the Ur-Slaanesh, following his kidnapping by ~~Elves~~ Aelves in Age of Sigmar (no seriously, that's actually canon). Luxscious and the other Keepers who decided they themselves were the only one worthy of that position had to play down their ambition as Archaon, now supreme boss of all Chaos (no, seriously), won't accept a pretender to a position that he himself wants to take...for some reason. Apparently being HNIC rapeviking means perfection and pride now. Well, the Vikings were actually pretty vain about their appearances, so it isn't that far fetched.
Manushya-Rakshasi A keeper of secrets brought forth during the Heresy by an Emperor's Children apothercary who dabbled in demon summoning as a hobby. Converted a ton of noise marines into demons and attempted to kill as many White Scars as possible, invading their ships whilst they journeyed in the Webway. Jaghtai Khan gutted it with it's own sword.