Sisyphus

"Sisyphus cheated death. First he chained up Thanatos, the reaper of souls, so no one could die. Then when Thanatos got free and was about to kill him, Sisyphus told his wife to do incorrect funeral rites so he wouldn't rest in peace."

- Nico di Angelo

Sisyphus (Σίσυφος in Ancient Greek) is a spirit in the Fields of Punishment.

History
Sisyphus promoted navigation and commerce, but was avaricious and deceitful, violating the laws of hospitality by killing travelers and guests. He took pleasure in these killings because they allowed him to maintain his dominant position. From Homer onwards, Sisyphus was famed as the craftiest of men. He seduced his niece, took his brother's throne and betrayed Zeus' secrets (one of which where he told the river god Asopus of the whereabouts of his daughter Aegina). When Sisyphus died, Zeus then ordered Thanatos (Death personified) to chain Sisyphus in Tartarus.

Sisyphus cheated death by imprisoning Thanatos when Sisyphus asked how the chains worked. No one could die or pass on. The Fates were furious because of this. Ares was mad that the battles had lost their fun because nobody lost their lives, and released Thanatos. Before Sisyphus died, he told his wife, Merope, not to bury him so when he was brought down to the Underworld, he defended his right to a proper funeral. Persephone let him go where Sisyphus scolded Merope for not giving him a proper funeral. However, Sisyphus was forcefully dragged back to the Underworld by Hermes when he refused to go back to the dead. The Judges told him to roll a boulder up a hill as his punishment, and when he finished that task, he would be freed. Sisyphus tried, but it fell back when he got close to the top of the hill. He tried again, and again, and again forever. This represented the punishment of Sisyphus claiming that his cleverness surpassed Zeus causing the god to make the boulder roll away from Sisyphus binding Sisyphus to an eternity of frustration.

The Lightning Thief
When Percy Jackson first entered the Underworld with Annabeth and Grover, after passing Cerberus, Percy sees Sisyphus from afar in the Fields of Punishment pushing his rock up the hill. The demigod sees Sisyphus so far away that he compares him as the size of an ant and his hill as the size of an ant hill.

The Sword of Hades
"I'll bust out of here soon, and when I do, they'll all be sorry!"

- Sisyphus

When Percy, Thalia and Nico go and try to find to find the thief of the Sword of Hades, they encountered Sisyphus. Thalia pushed the boulder for Sisyphus while Percy and Nico asks him about the thief and a possible exit of the Underworld. He does not really help them much, but he does say that he helped someone else. He said that he told that other person to go see Melinoe. When they got the information, Thalia unwillingly forces it back to Sisyphus who screams at them to let him free of his punishment which he believes it is just a minor set back. It gave him just one short break.

Trivia

 * The word "Sisyphean" is derived from his name, meaning "endless and unveiling, as a labor or task."
 * Nico nicknamed Sisyphus "Sisy" when asking him where the Sword of Hades went.
 * Sisyphus can speak several different languages.