Iphigenia was the eldest daughter of Clytemnestra and Agamemnon.
When in Aulis, Agememnon killed a stag in a grove sacred to Diana.
Angered, Diana stopped the winds so that the Greek fleet could not sail to Troy. The seer Calchas was called upon, and he announced that the only way the Greek fleet would sail was if Iphigenia was sacrificed.
Agamemnon at first adamantly refused, but, under pressure, Agamemnon slowly gave in and he agreed to the sacrifice.
Hyginus relates this story in Fabulae 69:
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Agamemnon cum Menelao fratre Achaiae delectis ducibus Helenam uxorem Menelai, quam Alexander Paris avexerat, repetitum ad Troiam cum irent, in Aulide tempestas eos ira Dianae retinebat, quod Agamemnon in venando cervam eius violavit superbiusque in Dianam est locutus. Is cum haruspices convocasset et Calchas se respondisset aliter expiare non posse, nisi Iphigeniam filiam Agamemnonis immolasset, re audita Agamemnon recursare coepit. Tunc Ulixes eum consiliis ad rem pulchram transtulit; idem Ulixes cum Diomede ad Iphigeniam missus est adducendam, qui cum ad Clytaemnestram matrem eius venisset, ementitur Ulixes eam Achilli in coniugium dari. |
Agamemnon with brother Menelaus, having been chosen leaders of Achaia, were journeying to Troy for the purpose of recapturing Helen, wife of Menelaus, whom Alexander Paris hauled away. In Aulis, vengeful Diana herself held back a storm because Agamemnon harmed her doe in hunting and had spoken against Diana rather arrogantly. When a soothsayer had convoked and Calchas himself had answered that he was not able to atone otherwise unless he had sacrificed Iphigenia, daughter of Agamemnon. Having heard this statement, Agamemnon began to refuse [to obey Calchas]. Then Odysseus, by council, brought him [Agamemnon] across toward the fair matter; the same Odysseus has been sent with Diomedes for the purpose of influencing Iphigenia, who when he had come toward the mother Clytemnestra, Odysseus pretended Iphigenia to be offered to Achilles in marriage. |
As Hyginus writes in his Fabulae, Iphigenia was taken away under the pretext that she was to marry the great warrior Achilles. Clytemnestra was overjoyed and readily sent Iphigenia to Aulis.
When Iphigenia arrived, she and Clytemnestra learned, to their horror, that there would be no marriage. Achilles was outraged at having his name used to deceive Clytemnestra and Iphigenia, and is shown below drawing his sword in anger against Agamemnon.

Achilles declared that he would protect Iphigenia, but his attempts to persuade the army to back him were largely unsuccessful (Gantz, 587). Despite his and Clytemnestra's protestations, Iphigenia was sacrificed.
In Euripides' play, Iphigeneia in Aulis, Iphigenia voluntarily agrees to sacrifice herself. In the play, she does this for she "imagines that she will win the fame for heroism denied to women in the real life of classical Athens," and she calls herself the "liberator of Greece" (Fantham et al., 122).
Some writers, such as Hyginus, write that Iphigenia was not killed, but that a great fog fell over the place of sacrifice and that Iphigenia was spirited away to become a priestess of Diana.
Hyginus writes in Fabulae 69:
| Quam cum in Aulidem adduxisset et parens eam immolare vellet, Diana virginem miserata est et caliginem eis obiecit cervamque pro ea supposuit Iphigeniamque per nubes in terram Tauricam detulit ibique templi sui sacerdotem fecit. | When he had brought Iphigenia into Aulis and the parent [Agamemnon] wished to sacrifice her, Diana pitied the maiden and threw a fog over them and she substituted a doe for Iphigenia and through the gloom carried Iphigenia away into the Tauric land and there she made her a priestess of her [Diana's] temple. |
Following this storyline, Orestes, after killing his mother, went to Tauris. There, he was reunited with Iphigenia and she went to become the priestess of Artemis Brauronia.
Whether she was sacrificed at Aulis or spirited off to become a priestess of Diana, Iphigenia was a tragic heroine. She was the first person to have her life flipped upside down because of the Trojan War and it was her death which allowed the Greek fleet to set sail for Troy.
Edward Tripp writes that "Iphigeneia was worshiped in at least one Greek city. Herodotus [4.103] reported that in his day the Taurians still offered human sacrifices to a virgin goddess who they said was Agamemnon's daughter. Most scholars believe that Iphigeneia was, in fact, a form of Artemis" (Tripp, 324).