Sisyphus

Sisyphus 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. When Sisyphus died, Zeus then ordered Thanatos (Death personified) to chain Sisyphus in Tartarus.

Sisyphus cheated death by imprisoning Thanatos, the Greek Grim Reaper. No one could die, the headless, bloodless, diseased, etc. couldn't 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. Sisyphus 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. Sisyphus tried, and it fell back. He tried again, and again, and again forever.

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 (Sisyphus was so far away he was the size of an ant and his hill was the size of an ant hill).

The Demigod Files
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. 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. Giving him just one short break.