Sacrifices.

You guys, (Annabeth, Grover, You, Percy, and Tyson) were on a hunt, and came across a few monsters. There was one last one, and it came charging at you. You handled it pretty well, until it started to get more aggressive. Annabety tried to help, but Percy.. He sacrificed to be injured for you to not be injured.

Grover: "..PERCY! DON'T DO SACRIFICE-"

too late. He already did, and he was badly injured.