King Pelias was the father of Acastus, Pisidice, Alcestis in Greek mythology. He was a son of Tyro and Poseidon.
Tyro was married to Cretheus (with whom she had one son, Aeson) but loved Enipeus, a river god. She pursued Enipeus, who refused her advances. One day, Poseidon, filled with lust for Tyro, disguised himself as Enipeus and from their union was born Pelias and Neleus, twin boys. Tyro exposed her sons on a mountain and they were raised by a maid. When they reached adulthood, Pelias and Neleus found Tyro and killed her stepmother, Sidero, for having mistreated her. Sidero hid in a temple to Hera but Pelias killed her anyway, causing Hera's undying hatred of Pelias.
Pelias was power-hungry and he wished to gain dominion over all of Thessaly. To this end, he banished Neleus and Pheres, and locked Aeson in the dungeons in Iolcus. While in there, Aeson married and had several children, most famously, Jason. Aeson sent Jason to Chiron the centaur, on Mt. Pelion, to be educated while Pelias, paranoid that he would be overthrown, was warned by an oracle to beware a man wearing one sandal.
Many years later, Pelias was holding the Olympics in honor of Poseidon when Jason, rushing to Iolcus, lost one of his sandals in a river while helping someone cross. When Jason entered Iolcus, he was announced as a man wearing one sandal. Paranoid, Pelias asked him what he (Jason) would do if confronted with the man who would be his downfall. Jason responded that he would send that man after the Golden Fleece. Pelias took that advice and sent Jason to retrieve the Golden Fleece.
During Jason's absence, Pelias thought the Argo had sunk, and this was what he told Aeson and Promachus, who committed suicide by drinking poison or were both killed directly by Pelias.
While Jason searched for the Golden Fleece, Hera, who was still angry at Pelias, conspired to make him fall in love with Medea, whom she hoped would kill Pelias. When Jason and Medea returned, Pelias still refused to give up his throne. Medea conspired to have Pelias' own daughters (Peliades) kill him. She told them she could turn an old ram into a young ram by cutting up the old ram and boiling it. During the demonstration, a live, young ram jumped out of the pot. Excited, the girls cut their father into pieces and threw them in a pot. Pelias did not survive.
Pelias had a son, Acastus, who later drove Jason and Medea to Corinth and so reclaimed the kingdom. His daughter Alcestis was the wife of King Admetus of nearby Pherae, who died for him and was later rescued by Heracles. He had another daughter named Pisidice.