When Cosmo learns the faun king is in the human realm. He panics. See, Silenus happens to be his fiance... and he's never told anyone that he ran away on his wedding day, leaving his king at the alter.
Silenus made a deal with the devil-- the queen of the fae-- when he learned the human realm was going to be closed, keeping him separated from his mate and betrothed. He has to track down Cosmo and convince him that they are meant for each other, even though his stubborn fiance seems determined to challenge him at every turn.
Cosmo doesn't know Silenus's new status, though, and the hunt is now on. Silenus is facing a dark and dangerous path, and he wants nothing more than his consort at his side.