The Defeated the Third Sister of the Void is a collection that is started and completed in the Voidsisters storyline given in the Witches of Dathomir Theme Park.
The storyline starts from different NPCs, depending on which faction you are supporting.
Nightsister - Gethzerion
Singing MountaiDefeated the Third Sister of the Voidn Clan - Aujante K'lee
The collection is started and completed with the completion of the Dead is Dead quest.
Completion of the collection rewards the player with the Third Sister of the Void Badge.