N'Kari is either a Keeper of Secrets or a Daemon Prince, depending on if you're reading newer or older fluff. She/He (either works, but since Keepers of Secrets are hermaphroditic, it doesn't matter) is notable for appearing in the canon before his/her backstories even happened, as well as swapping her/his Warhammer Fantasy and Warhammer 40k backstories around between the two universes and thus is the primary evidence of a multiverse connected to the Warp after the map of the Realm of Chaos.
N'Kari's name is taken from the villain "Nakari", an evil and seductive queen with sorcerous powers from the Soloman Kane novel "The Moon of Skulls" by Robert Howard (yes, the same guy who wrote Conan the Barbarian).
The Legend (Warhammer Fantasy)
In the Old World, N'kari was never mortal and instead a powerful Keeper of Secrets from primordial times that fought Aenarion the Defender a couple of times, then was struck down by him in the last battle and trapped in the fabric of a magical vortex created by Caledor the Dragontamer and friends to block Chaos from the World. She eventually got out, and to her props even Malekith was on edge about the whole thing. She proceeded to wreak unspeakable carnage in Ulthuan, hunted Aenarion's descendants down to wipe them out and eat their souls as revenge, and tried to destroy the Shrine of Asuryan, but that last one didn't work out too well.
As a result, N'Kari has played Saturday morning cartoon villain to the T&T twins (Tyrion and Teclis) ever since, appearing and taunting them, getting an asswhooping (and a corpse-humping from Tyrion most likely), then popping back into the Warp while shrieking about how he will be back and they won't be so lucky next time until the next time actually comes which ends up as just a rinse/repeat of the last time. Malekith did get N'Kari to hunt down Tyrion and the Everqueen for him, but a last minute rescue from Teclis saved them and banished N'Kari again. Since Malekith is Aenarion's son, N'Kari did try to go after him once in Naggarond, but Malekith kicked her ass (even without dragon help) and she never went after him again.
He also made an appearance in Warhammer Online standing in a fountain staring at the sun, but whatever.
According to latest studies, N'Kari once was a human native of Old Earth, an actress of dubious morals going by the 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 the Warp, which was the formation of Slaanesh, and immediately recognized it as her future master. N'Kari then patiently waited for the birth of the new Chaos God and totally and undoubtedly gave 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 than the All-Mighty Corpse Emperor, and was sent back to Slaanesh to have a rape party afterwards. She 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. Fortunately, Drastanta showed up in the nick of time and slew N'kari with his mighty lance before going all emo and go sulk forever.
She also was doing some scavenging on the derelict Craftworld of Malan'tai, where she was "Om Nom'ing" the Eldar Spirit Stones, until some of those retarded SUPER KNIGHTS OF THE EMPEROR swooped in, curbstomped her demonic servants, and 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 days (though one wonders what would happen if she saw this page?).
It is rumored that the Fifty Shades trilogy were some of her daydreams kept in
(she/he/shim?) its diary before N'kari's ascension to daemonhood, and that foul heretic E. L. James (who is secretly an ardent devotee of Slaanesh) found them and decided to spread them in preparation for the corruption of humanity by She-Who-Thirsts. The Inquisition has a standing order that all copies of these obscene texts (and anyone who thinks they are actually novels and not the steaming piles of foul heresy they are) must be shot into the heart of the nearest star straight away. Unfortunately, N'kari has grown even more powerful as there is now a trilogy of Fifty Shades of Grey movies. Appallingly, her literary musings are neither well-written nor interesting nor even particularly deviant. The sales figures are conversely monumental, which is only further overwhelming evidence that the tomes are suffused with Slaaneshi magics.
She turns up in the new Horus Heresy book Slaves to Darkness as the consort/plaything for the newly-living-in-the-EyeofTerror Fulgrim. She assumes a fat slob parody of Fulgrim (think snake tail, 4 arms, Fulgrim's face and hair but fat). After Lorgar binds Fulgrim with his true name, N'kari gives him one of her black fangs as a means of semi-permanently binding her.
N'Kari is Slaanesh's leading legendary lord and OH BOY he's powerful. If selected as the player faction for the campaign, the Advisor delves into his pleasure den to offer a means to traverse the Chaos Maelstrom to reach the captive (and dying) bear God Ursun in exchange for a drop of Ursun's blood. N'Kari, wanting to sample the final, sorrow filled moments of the God's life, agrees quite readily to the Advisor's request.
He is able to seduce enemy units to fight for him in the battle, vassalize all human and ELF factions plus be a very fast hero-killer! As a Legendary Lord, his unique skill tree is based on increasing his performance, allowing him to debuff an enemy to the point that even Skarbrand won't be able to hold the line against him if N'kari debuffs him. He is also a caster with the Lore of Slaanesh. His faction effects and a bit of his skill tree increase his diplomacy with Chaos, Human and Elf factions. And he is the only monogod chaos faction that can engage in proper diplomacy with order factions (not Skarbrand's fault an axe to the head doesn't count as a proper greeting).
In the Realms of Chaos campaign you start in the western corner of the Chaos Wastes, so you have the western part completely secure, so you can focus on expanding. The bad part in this deal? Skarbrand starts right to the east of you. Chances are, you'll meet him as you wrest control of your province from a minor Khornate faction. And more than likely, he is already on his way, since his expansion road leads directly to you. Prepare accordingly. The campaign slows down a fair bit once Skarbrand is dead. You have Norsca to the south to either conquer or vassalize, there is a minor Khornate faction near Skarbrand's province, and you have to remember that Kislev will appear on the radar sooner or later. In Immortal Empires, N'kari starts in Ulthuan, ready for another scrap with the twins. He starts in Chrace, near Eltharion and Allariele. With no close allies, you'll have to play this one with survival on mind. Consider emmigrating to the Chaos Wastes if things go sour. And for all that is holy, do NOT let Allariele consolidate her grasp on Nagarythe, or nobody will save you from the murder pidgeon with the Sword of Khaine. You have been warned.