In the south Remnants of Elkrim Shrine, there is Leinad's Stone Coffin. Click on the stone coffin to summon Leinad and defeat it. Then, talk to Captain Phoebe at the Algon Post.
Captain Phoebe: This scroll is designed to bring back Leinad from his Captain Phoebe: sleep. That man Alden was carrying something Captain Phoebe: very dangerous. NakanoAzusa: What should we do? Captain Phoebe: The answer is simple! You use this scroll to wake up Captain Phoebe: Leinad! HiiragiKagami: That's absurd! How could my master defeat a monster HiiragiKagami: that was hard to seal for advanced mages?! Captain Phoebe: Don't worry! Even if you wake up Leinad with this scroll, Captain Phoebe: he wouldn't even have one tenth of the power he used to Captain Phoebe: have. That's because the ceremony hasn't been Captain Phoebe: performed yet. The Dirty Dozen was preparing one to Captain Phoebe: bring back Leinad's power. NakanoAzusa: So, it's not as strong as it used to be? You think I would NakanoAzusa: be able to defeat it? Captain Phoebe: Of course! Even the small power of Leinad would mean Captain Phoebe: incredible power for us Aitan. If we just leave it as it is, Captain Phoebe: the Dirty Dozen will wake up not just Leinad, but also Captain Phoebe: the other two monsters. It would be too late to regret Captain Phoebe: then! NakanoAzusa: I see. Captain Phoebe: Go to Leinad's stone coffin and wake the monster Captain Phoebe: up, then put him into eternal sleep. That's the only Captain Phoebe: solution we have!
Reminder Dialog
Captain Phoebe: Just be careful. I don't know how a Priest would match Captain Phoebe: up against Leinad.