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.
Originally a High Elven maiden during the first Chaos invasion, her parents sacrificed her to Slaanesh in exchange for protection from the Slaaneshi Daemons that were overrunning Ulthuan. Slaanesh agreed to the pact, and (because in Fantasy Slaanesh is a bit less consumption-based in regards to his Keeb policies) he returned her to the mortal world empowered as a Greater Daemon. She immediately slew her parents, who were then consumed by Slaanesh.
From that point on, Dechala was given free reign to do as she saw fit in Slaanesh's name. She was banished with the rest of the Daemonhost after Aenarion and Caledor the Dragontamer established the Vortex and Waystone network, after which point she waited for a chance to leap back into the world and wreak mischief.
Unfortunately, her position of primary Slaaneshi Daemon 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 is Slaanesh's now (though Morathi would eventually get free by the time Age of Sigmar came around), then fights Tyrion after he gets turned into the Incarnate of Light. She gets killed by him, likely returning to the Realm of Chaos like any other Daemon. She has yet to appear in Age of Sigmar, where the loss of Slaanesh has resulted in a clusterfuck civil war of Chaos between loyalist Daemons, rebel Daemons, and those loyal to Archaon.
Dechala 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.
In terms of Lore, Dechala leads the Tormentors, a Slaanesh WHFB 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.
On the Tabletop
Dechala hasn't had a model since the golden age of blocky metal models that caused cancer and chipped when sneezed on. 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...