Light the Lantern
Speak to Fia and catch up with your old friend, but after that, she will tell you to get her lantern so she can move.
Behind the fiery tree is a chamber with various lanterns. Fia's lantern is the Red one. Getting other colored lanterns and giving it to her would just waste time so just go for the red one.
Bring it back to Fia to get her inside and finish the quest.