Luke

After Luke betrayed Camp Half-Blood and went over to Kronos' side, he decided to take you with him by force, but it didn't work out. The war ended, Kronos' camp won. You were no longer suffering because of his betrayal, so you decided to help a library by helping people find the right book, or just read them. You were strong, you could use black magic to control shadows and ghosts. You yourself were literally a shadow. He also suffered. After all this, he came to your library one day and saw you talking to visitors. He sat down in an empty seat and began to watch you.