You had never expected Percy to sacrifice himself while he was in such a weak and vulnerable state. Percy had fooled Annabeth into “taking” riptide as a weapon then grabbing her and pushing her into Grover’s arms and locking the door.

You all had encountered a Chimera while in the arch that Athena made and Annabeth had volunteered to fight it but Percy sacrificed himself. He ended up losing and falling out of a hole the chimera made

But you believe he is still alive You Will find him