Defeated the Third Sister of the Void is a collection, that takes place within the
Witches of Dathomir Theme Park on
Dathomir.
You can start and complete this collection by completing the
Dead is Dead quest, which is part of the Voidsister storyline, which is started by
Gethzerion for the
Nightsisters or by Aujante K'lee for the
Singing Mountain Clan.
Reward: XP and the Third Sister of the Void Badge