Dechala the Denied One is a Chaos Champion from Warhammer Fantasy Battle, being a High Elf who converted to worship of Slaanesh. She was one of the original Chaos special characters to be released back in the day for Warhammer as part of the Champions of Chaos splatbook.
The Denied looks like an angry Elven maiden with six arms, milky white skin, a giant snake's body from the waist down, multiple sting-bearing tailtips that secrete a euphoria-inducing corrupting venom, and glowing deep blue eyes.
Originally a High Elven maiden during the first Chaos invasion, Dechala’s parents sacrificed her to a daemon prince of Slaanesh in exchange for protection from the Slaaneshi Daemons that were overrunning Ulthuan. The daemon prince (called Samael in GWs boundless creativity) agreed to the pact, but did not devour his price. Instead, he tutored her in the ways of Slaanesh and in time, she became a Chaos champion of her own. In time, she became angry at her spouse, who then complained before Slaanesh that his waifu had turned from him. Slaanesh reprimanded his servant, but offered a compromise: If she would return to him on her own volition, Samael would be allowed to do with her as he pleased. As long as she didn't and Samael still lived, she would never ascend to daemonhood. As a result, Dechala murdered her way across the Chaos Wastes, but daemonhood was eternally denied to her. She now leads the Tormentors, a Slaaneshi warband. Her minions actively take slaves and create drugs from the bodies of both the living and the dead, including one elixir so intoxicating and deadly that Dechala uses it to enslave people as it gradually twists them into that which must not be named. Though people have been saved from the Elixir's effects, the odds are against it, since so much as a drop is capable of making an individual into an addict. Because Dechala always needs slaves, her forces are eternally on the hunt.
Unfortunately, her position of primary Slaaneshi character was usurped by The Masque, and to a larger degree N'kari due to N'kari being her 40k counterpart and in GW's days of neglecting Fantasy everything that worked in the grimdarkness blahblahblah was just pasted back into Fantasy. Dechala was brought back for End Times, where she player a minor role in attacking the Druchii. She makes a cameo in the last End Times novel where she mocks Malekith by throwing Morathi's staff at his feet and claiming she fully belongs to Slaanesh now, then fights Tyrion after he gets turned into the Incarnate of Light. She is killed by him and is was presumed dead, however she made an unexpected return in Age of Sigmar with an audio drama named after herself; “The Denied” reveals she has finally become a full fledged daemon and is now regarded as an ancient and powerful servant of the Dark Prince and worshipped by mortals as “She Who Was Denied”.
On the Tabletop
Slaanesh champions tend to be quick 'n' choppy, and that tradition definitely started with Dechala; eight inches of movement, and then she's tossing out six attacks with Weapon Skill 8(!) and Initiative 10(!!), all of which are hitting at Strength 5 and due to a special rule prevent those wounded but not killed by her from being able to attack her for the rest of the game? Holy shit... She also has Hatred for all Khorne daemons and mark-bearers, and her "Dances of Slaanesh" give her a couple of special options, including imposing a -1 on To Hit rolls against her, giving herself a +1 to her own To Hit rolls, or trading 2 attacks of her own to automatically parry (and thus negate) one enemy attack per two she gives up. She admittedly can't take much punishment (only 2 Wounds), but at Toughness 5, that's still not as easy to strip away without cannonballs to the face.
One probable reason why she hasn't resurfaced is because of the complexity of her rules (doubtful, given Archaon and Nagash are still playable). Another is because "six-armed sword-waving battle-dancing lamia" is an image pretty damn close to the Marilith, a demon from Dungeons & Dragons, and GW fears accusations of copyright infringement if they use her. Then again, Square Enix gets away with it. And now that Morathi and the Daughters of Khaine displaced her with the Lamian look, chances are slim of any official return in plastic.