Agamemnon kills a deer by accident in Aulis, while him and his crew were preparing for war.
After he kills the deer he gets worried because he now knows that Artemis, the goddess was not going to be happy with the sin he has committed and now there was going to be a consequence.
Now he gets the news about his consequence. The sacrifice that the goddess wanted from him was to kill his oldest daughter Iphigenia.
Now that Agamemnon knows that the goddess wants his daughter to be killed, he sends his wife Clytemnestra and his eldest daughter Iphigenia a letter that says she will marry a worrier named Achilles.
When Clytemnestra and Iphigenia arrive, Iphigenia was really exited until Agamemnon her father breaks the truth to her. He tells her that she isn't getting married and that he wanted to tell her that the goddess wanted her dead.
After Achilles appears, he tells the mother that he didn't say he wanted to marry Iphigenia, and now everyone knows that it was a trick. So Iphigenia decided that it was the best option for everyone if she sacrifices her self.