Admetus（or Admhtos), a son of Pheres, the founder and king of Pherae in Thessaly, and of Periclymene or Clymene. (Apollod. 1.9.2, 9.14.)
He took part in the Calydonian chase and the expedition of the Argonauts. (Apollod. 1.9.16; Hyg. Fab. 14. 173.) When he had succeeded his father as king of Pherae, he sued for the hand of Alcestis, the daughter of Pelias and Anaxibia. Homer (Hom. Il. 2.715) calls Alcestis the fairest among the daughters of Pelias.
Pelias, in order to get rid of the numerous suitors, declared that he would give his daughter only to the man who should come to his court in a chariot drawn by lions and boars.
This task Admetus performed by the assistance of Apollo, who served him according to some accounts out of attachment to him (Schol. ad Eurip. Alcest. 2; Callim. h. in Apoll. 46, &c.), or according to others because he was obliged to serve a mortal for one year for having slain the Cyclops. (Apollod. 3.10.4.)
On the day of his marriage with Alcestis, Admetus neglected to offer a sacrifice to Artemis, and when in the evening he entered the bridal chamber, he found there a number of snakes rolled up in a lump.
Apollo, however, reconciled Artemis to him, and at the same time induced the Moirae to grant to Admetus deliverance from death, if at the hour of his death his father, mother, or wife would die for him. Alcestis did so, but Kora, or according to others Heracles, brought her back to the upper [living] world. (Apollod. 1.9.15; compare ALCESTIS.) Alcestis’ sacrifice for Admetus was highly celebrated in antiquity.
Towards her father, too, she shewed her filial affection, for, at least, according to Diodorus (4.52 ; comp. however, Palaeph. De incredib. 41), she did not share in the crime of her sisters, who murdered their father.
Ancient as well as modern critics have attempted to explain the return of Alcestis to life in a rationalistic manner, by supposiung that during a severe illness she was restored to life in a physician of the name of Heracles. (Palaeph. l.c. ; Plut. Amator. p. 761.) Alcestis was represented on the chest of Cypselus, in a group shewing the funeral solemnities of Pelias. (Paus. 5.17.4.) In the museum of Florence there is an alto relieve, the work of Cleomenes, which is believed to represent Alcestis devoting herself to death. (Meyer, Gesch. der bildend. Künste, i. p. 162, 2.159.)
Source stories about Admetus and Alcestis
Apollodorus, The Library. 1.9.15:
 When Admetus reigned over Pherae, Apollo served him as his thrall,1 while Admetus wooed Alcestis, daughter of Pelias. Now Pelias had promised to give his daughter to him who should yoke a lion and a boar to a car, and Apollo yoked and gave them to Admetus, who brought them to Pelias and so obtained Alcestis.
But in offering a sacrifice at his marriage, he forgot to sacrifice to Artemis; therefore when he opened the marriage chamber he found it full of coiled snakes. Apollo bade him appease the goddess and obtained as a favour of the Fates that, when Admetus should be about to die, he might be released from death if someone should choose voluntarily to die for him.
And when the day of his death came neither his father nor his mother would die for him, but Alcestis died in his stead. But Persephone sent her up again, or, as some say, Hercules fought with Hades and brought her up to him.2
1 Apollodorus, The Library. 3.10.4: But Zeus, fearing that men might acquire the healing art from him [Aesculapius, hero and god of medicine] and so come to the rescue of each other, smote him with a thunderbolt. Angry on that account, Apollo slew the Cyclopes who had fashioned the thunderbolt for Zeus. But Zeus would have hurled him to Tartarus; however, at the intercession of Latona he ordered him to serve as a thrall to a man for a year. So he went to Admetus, son of Pheres, at Pherae, and served him as a herdsman, and caused all the cows to drop twins.
2 This story is immortalized by Euripides in his noble tragedy Alcestis, happily still extant. Compare Zenobius, Cent. i.18, which to a certain extent agrees verbally with this passage of Apollodorus. The tale of Admetus and Alcestis has its parallel in history.
Once when Philip II of Spain had fallen ill and seemed like to die, his fourth wife, Anne of Austria, “in her distress, implored the Almighty to spare a life so important to the welfare of the kingdom and of the church, and instead of it to accept the sacrifice of her own. Heaven, says the chronicler, as the result showed, listened to her prayer. The king recovered; and the queen fell ill of a disorder which in a few days terminated fatally.” So they laid the dead queen to her last rest, with the kings of Spain, in the gloomy pile of the Escurial among the wild and barren mountains of Castile; but there was no Herakles to complete the parallel with the Greek legend by restoring her in the bloom of life and beauty to the arms of her husband.