Tharasdun

Tharazdun burned our world, destroyed seventy percent of the population and created the Hallowed all in seconds." - Clerical Notation 53 on the Night of Sorrows

The Elder God of Chaos
Created the Gods of Darkness, Though covered up by Ao, he had a hand in creating much of the Realms. 
Sealed away by Ao for an eon. Freed by the Chaos Crew and the Theras. He unleashed upon the world FiendFyre during the Night of Sorrows

He was defeated at Khisfire Necropolis, by the Chaos Crew on 1089AC