Branwen Rescued, also known as Freeing Branwen is a companion quest that appear in journal instead of quests tab.
Zeke offers to sell you such a scroll for 500 , but you can pickpocket him for it instead or kill him, although that results in reputation loss. You can also charm him and use a scroll from his quick slot. You can also buy these scrolls at most temples for much less gold, the nearest being he Temple of Helm in Nashkel.
Once you have a scroll, cast it on the statue and speak to Branwen. She will offer to join your party as a reward.