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 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 Great Vortex and Waystone network, after which point she waited for a chance to leap back into the world and wreak mischief.
The End Times
Dechala was brought back for The End Times, where she play 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, 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.