In The End, It's Blackthorn Blood

When the return of an already fulfilled prophecy intertwines the heroes who saved Olympus with the Shadowhunters of the Los Angelos Institute, secrets about the Blackthorns are revealed that even they didn't know about themselves. Secrets about a certain figure in their family who they knew barely anything about until they met the demigods. New worlds are opened and a new problem arises, one that both teams have never seen before.

All characters belong to Cassandra Clare and Rick Riordan