Frank Zhang's life was tied to a piece of wood. If it burns up, he will die. It was given to him by Juno due to him "having too many gifts." It is supposed to be a huge weakness to balance out Frank's many advantages and gifts.
Frank asks for it back after Leo gave him a fire-proof bag to keep it in.
By the time of Caligula’s Assault on Camp Jupiter, Frank has come to see the firewood as a part of him, however he has become reckless. During the Battle of San Francisco Bay, he ignites the stick to kill Commodus and Caligula. Miraculously, the son of Mars does not die when the stick burns up.
- This idea comes from the myth of Meleager; In Greek mythology, Meleager was a hero and famed host of the Calydonian Boar Hunt. When Meleager was born, the Fates predicted he would only live until a brand, burning in the family hearth, was consumed by fire. Overhearing them, his mother immediately doused and hid the brand. When she found out that Meleager had killed her brother for the love of the huntress Atalanta, she placed the brand upon the fire, thus fulfilling the prophecy and killing Meleager. 
- In The Son of Neptune, the goddess Iris told Frank that he would die holding his stick, watching it burn.
- This somewhat came true in The Tyrant’s Tomb, when Frank burns it to take out two of the emperors but services, symbolically dying to free himself of the stick and being reborn to a life not bound to it.