This actually happens before the beginning of the story we see onstage, but it is crucially important to everything that follows. Not only is this what gets Orestes moving on his revenge plot, but he constantly refers to the oracle for reassurance as he moves towards his ultimate confrontation with the monster: his mother Clytemnestra (plus Aegisthus, but nobody really cares too much about him).
As soon as he gets back to Argos, Orestes finds out that he has allies. With a little help from these new-found friends, Orestes is able to make his way into the palace and kill the detestable Aegisthus. Everything looks like it's going swimmingly.
After killing Aegisthus, a minor-baddie who doesn't really count for much, Orestes finally finds himself face-to-face with the mother (hehe) of all his troubles. Clytemnestra makes a bid for sympathy, and Orestes wavers in his determination.
Pylades's words of advice got Orestes out of the Frustration Stage, but this has only geared him up for the final verbal battle with Clytemnestra, in which he must not lose his cool – or else he will lose his opportunity for revenge.
Orestes finally has enough of the argument and drives his mother inside the palace to be killed. Moments later, he reemerges, displaying the bodies of his two victims. It looks like the monster is slain. But then, a new set of monsters appears: the horrible Furies of vengeance. Even though it isn't clear whether these fearsome ladies really exist (they could just be the products of Orestes's own troubled mind), their effects on our hero are all too clear. In this way, Libation Bearers ends by setting us up for the sequel: Eumenides.