Thoas is king of the Taurians in the Crimea when Iphigenia serves as priestess of Artemis and must sacrifice trespassing Greeks. Euripides' Iphigenia among the Taurians makes him the ruler tricked by the siblings' purification ruse until Athena stays his pursuit. Nicander (via Antoninus Liberalis) makes him son of the river Borysthenes; Hyginus identifies him with the Lemnian Thoas — a merge the atlas rejects by keeping thoas-tauris separate from thoas-lemnos.
Adapted from Wikipedia, "Thoas (king of the Taurians)" · CC BY-SA 4.0
I.Identity and parentage
Thoas rules the barbaric Taurians when Agamemnon's daughter Iphigenia, rescued from Aulis by Artemis, becomes the goddess's priestess and is forced to sacrifice Greek strangers. Antoninus Liberalis, citing Nicander, makes him son of Borysthenes, the great northern river (the Dnieper).
Hyginus and Valerius Flaccus sometimes identify him with Thoas of Lemnos, father of Hypsipyle; Grimal, Tripp, and Smith treat the kings separately. The atlas follows the separate tradition: thoas-tauris is not thoas-lemnos.
II.Iphigenia among the Taurians
In Euripides' play, Orestes and Pylades arrive and are captured for sacrifice. When Iphigenia and Orestes recognize each other, Orestes would kill Thoas, but she prefers deceit: she tells the king that the matricide pollutes the victims and they must be washed in the sea, taking the sacred image as well. Thoas believes her; a messenger later reports the escape, and he orders pursuit until Athena intervenes and bids him let them go.
Hyginus ends the chase on the island of Chryses, son of Agamemnon and Chryseis: once Chryses learns he is half-brother to Orestes and Iphigenia, he kills Thoas.