The Last Olympian

Since the gods didn't help the demigods they had to protect Manhattan by themselves while the gods fight Typhoon. The first battle was about to start it was almost night and Kronos always attacked at night I don't know why but he did. Every single demigod spread to put bombs where Kronos army was going to pass. Annabeth and her friends were fighting other evil demigods Percy went to help them in he fight since Percy had the Achilles curse he was indestructible but suddenly a evil demigod went from behind and was about to stab him on his weak spot but Annabeth took the stab for him. My question is how or why did she take the stab.