Ascension Quest: To become a War Witch, you must first ascend into the role of a Enchantress or Valkyrie. Once you have completed the requirements for either of these classes, the Sisterhood will ask you to free Sister Talya from an evil warrior named Kaugris. Travel to the Forbidden Lands, defeat Kaugris to set Sister Talya free, then return to the guild for your final quest.
For your final task, the Sisterhood will tell you about a dread witch named Greuvenia the Pox who is summoning an army of monsters within the Vale of Ruin. Travel to the vale, defeat the witch and her abominations, then return to the guild to ascend into the War Witch role.