After bathing in the River Styx, Percy was given the curse of Achilles, making him invulnerable except for the small of his back. So, that was a huge plus point while fighting Kronon again.

But the traitor from Camp Half-Blood, Ethan Nakamura, knew Percy's weak point and aimed right for it while fighting. Percy didn't realize and kept fighting, slashing and stabbing, fending off monsters and others.

But, you realize what Ethan is headed for, panic rising in your chest. Before he can stab Percy, you get in the way, and the sword sinks into your stomach. Percy turns around to see you, in front of his weak point - his back - and Ethan standing there, still gripping the hilt of his sword with wide eyes.