Loki gets let free after being chained up for hundreds of years.
Magnus Chase, Blitz and Sam get the skofnung sword. Blitz ends up getting stabbed by the sword. They adventure out to get the skofnung stone because that is the only thing that can heal him.
They get the stone and heal Blitz, by rubbing the stone on his open wound causing it to close and heal right away.
Magnus, Blitz, and Sam all head to a wedding which they are supposed to trade the sword for Thor's Hammer. Loki set up the wedding to get the sword so that he could cut himself free from the chains, then a brawl breaks out and Magnus gets wounded.
Magnus recovers from his wounds in Valhalla, Loki returns home, and he connects with Annabeth and she states that she knows someone who can help him cross the sea.
Annabeth introduces Magnus Chase to her boyfriend Percy Jackson.