Hexed Staff is a quest available after Termini Cliff has been completed for the first time. It is part of the main quest line and becomes unavailable after it has been completed.
To complete the quest, Morrigan's Avengers need to be killed in Termini Cliff. Kills give Hexed Staffs, the quest is completed once 4 staffs are collected. At least two dungeon runs are needed to pass the quest.
Click on [Campaign] -> Select [Termini Cliff] -> Kill [Hexed Priests] to collect [Hexed Staff] x4. The once sacred priests have become puppets of evil and now even their holy staves have been sullied.