Iphigenia
Iphigenia was the favorite daughter of Agamemnon and Clytemnestra. As a result of an unwise vow at Aulis, Agamemnon was bound to sacrifice to Artemis the first living being who greeted him on his return home. Iphigenia ran out to meet him, and thus doomed herself.
Agamemnon returned to Aulis and deceived Iphigenia and her mother as to his intentions by sending for her to marry Achilles. Once she arrived in Aulis, he sought to sacrifice her, but she was taken up by Artemis and a deer substituted for the sacrifice. Clytemnestra's rage at Agamemnon over his attempt to sacrifice Iphigenia led to her infidelity with Aegisthus and their murder of Agamemnon.
After her rescue, Iphigenia was delivered to Tauris, where she served as a priestess whose duty it was to sacrifice foreigners. Much later, Orestes and his loyal ally Pylades arrived at Tauris as part of expiating his guilt after killing Aegisthus and Clytemnestra.
As they were foreigners, it was Iphigenia's duty to sacrifice them both, but she sought to secretly free one so that he could take a letter back to Greece on her behalf, begging her family to rescue her. Orestes and Pylades argued over which should be freed, with Orestes prevailing and ordering Pylades to go. As Iphigenia prepared to sacrifice Orestes, he and Pylades read the letter and realized that she was in fact Orestes' long-lost sister.
On learning the truth, Iphigenia escaped with the two of them, taking with them the statue of Artemis which they had sought. She returned to Mycenae with them and remained a priestess to her death, still engaging in human sacrifice.