Tantalus is a Greek demigod king who was sent to the Fields of Punishment for trying to feed the gods human flesh. He also temporarily served as activities director of Camp Half-Blood during the events of The Sea of Monsters. Mark Hamill was cast to play Tantalus, but was never seen in the film adaptation of The Sea of Monsters. 
Tantalus was the king of Tantalis, an ancient western Anatolian city he was named after. His father, Zeus offered him a spot at the Olympian table and he accepted. When he went to them, he asked to take nectar and ambrosia, back to his people. Zeus refused, and he was enraged. In order to humiliate the gods, he cooked his son Pelops and served it to the gods. Only Demeter touched the food because she was distraught about her daughter, Persephone. She didn't know what it was and ate part of Pelops' shoulder. Zeus took the parts of the boy, put them together, and was able to bring him to life once more. He was given a bronze shoulder made by Hephaestus. In another myth, he stole a golden dog, made by Hephaestus for Zeus, and gave the dog to his friend for safekeeping and later denied ever seeing it.
For his terrible crimes, Tantalus was destroyed by his father Zeus, who then personally took his soul to Hades in the Underworld. Hades subjected Tantalus to eternal starvation and thirst. He was made to stand in a lake with a fruit tree growing on the shore with fruit growing on a branch above him. Whenever Tantalus tried to drink, the water dried away and when he wanted to eat, a strong wind blew the fruits out of his reach.
As an addition, all male descendants of Tantalus were cursed whereas they are predestined to get murdered by a family member. This curse then continued straight to Agamemnon, the king of Mycenae and his family.
Tantalus replaces Chiron as the activities director for a time at Camp Half-Blood when Chiron is suspected of poisoning Thalia's tree. He seems to take particular pleasure when the campers realize who he is, and tells them a story about a mortal king who was beloved by the gods - himself. He doesn't like the campers and appeared to take particular pleasure in torturing Percy Jackson and his half brother Tyson, who is a Cyclops. He seemed to have a soft spot for Clarisse La Rue and was the one who decided to send her on the quest. He also started up chariot racing again with the warning that if anyone killed another person, they would not be able to have s'mores at the camp fire for a week.
When Stymphalian Birds attack the campers whilst they are watching the chariot race, Percy and Annabeth Chase decided to try and fight them off by using Chiron's boom box from the Big House. Clarisse tries to fight them unsuccessfully, but the birds are scared away by the noise of Chiron's collection of CDs and get shot down by the archers of the Apollo cabin. Tantalus congratulates Clarisse on winning the race, but then punishes Percy and Annabeth for being trouble makers. In Tantalus' mind, the Stymphilian Birds were simply minding their own business until they were disturbed by Annabeth and Percy's bad chariot racing and awful music that they played, therefore, he assigns them to a week of kitchen duty.
While he was at the camp, Mr. D took particular pleasure in watching him try to eat or drink, always unsuccessfully. Just before he was sent back to the Underworld, he managed to pick up a burger, but before he could eat it, he was sent back to the Underworld after being dismissed by Dionysus, much to the joy of the campers.
He is described as pale thin man in a threadbare orange prisoner's jumpsuit. He has blue shadows under his eyes, dirty fingernails, and badly-cut gray hair.
Tantalus is shown to be a cruel and sadistic person. He takes joy in watching others get hurt, such as when he was watching the Chariot Races. He also punishes others for his mistakes, as he had Percy, Annabeth, and Tyson do dishes for defending themselves against the Stymphalian Birds he set lose in the campers.
- Tantalus also makes a brief appearance in Percy Jackson's Greek Gods.
- Due to his egregious crimes Tantalus is the only demigod son of Zeus that his father personally destroyed with his Master Bolt, and then personally brought his soul to Hades for eternal torment.
- The word "tantalize", meaning to tease or disappoint by promising or showing someone something desirable and then withholding it, derives from Tantalus, due to the nature of his punishment.
- He was named after the city he was born in, Tantalis.
- Interestingly, in some mythology he was, through his son Pelops (the same son that was murdered and brought back to life), the ancestor of Hercules, and a relative-by-marriage to the first Perseus (at least two of Pelops' daughters ended up married to sons of Perseus).
- Tantalus' curse is the subject of the drama "Iphigenia in Tauris" by the German poet Johann Wolfgang von Goethe whereby Iphigenia and her brother Orest are able to overcome the bane through their encounter.
- The chemical element Tantalum (symbol Ta, atomic number 73) was named after Tantalus. The chemical element Niobium (symbol Nb, atomic number 41), derives from his daughter, Niobe.
- It was rumored that Mark Hamill, best known for playing Luke Skywalker in the Star Wars films, was going to play Tantalus in the film adaptation.