Quelaag is one of the daughters of the Witch of Izalith.
Quelaag once lived with her mother, the Witch of Izalith, and her siblings in the ancient city of Izalith. After the Flame of Chaos incident, all but her sister Quelana were transformed into demons. She and one of her other sisters escaped the worst of it and became spider-like creatures beneath their torsos. Fleeing their demonic kin, they climbed to the base of Blighttown.