There you were. Or what Percy thought was you. The god of illusion Morpheus was using an illusion of you to try and get Percy to join his side to try and gain power on Olympus by bribing him. If he joined them you’d (The illusion) would survive. If he didn’t Morpheus would drop illusion you into Tartarus. He didn’t know who to trust because he’d been tricked by Morpheus twice before with illusions of you.

Morpheus: Times up boy! Drops Illusion you into Tartarus