The group, Percy, Grover, and Annabeth, travel west to go and meet Hades, although the god who has turned was actually Ares. Percy later finds Zeus' lightning bolt in a backpack given to him by Ares. He returns it to Zeus, as well as Hades' Helm of Darkness, which was also in the possession of Ares.
For some more informations, http://riordan.wikia.com/wiki/The_Lightning_Thief
