She allows access to the Forgotten Temple.
During Black Moon, talk to her to join the event. Then, submit 3 or 7 Seal of Excavation in order to receive rewards from her.
Chapter 5 requires players to show an Evolution Stone (can be bought or borrowed) to the Chief, then to the General Van Handel, then to Petra. The chapter also requires three Seal of Excavation to be submitted to Petra before choosing a Sign from the General and receiving a Path Coin.