Alecto is one of the Erinyes, or Furies, in Greek mythology. According to Hesiod, she was the daughter of Gaea fertilized by the blood spilled from Uranus when Kronos castrated him. She is the sister of Tisiphone (Vengeance) and Megaera (Jealousy). Alecto is the Erinys with the job of castigating the moral crimes (such as anger), especially if they are against other people. Her function is similar to Nemesis, with the difference that Nemesis's function is to castigate crimes against the gods.
In the Aeneid (Book 7), Alecto was demanded by Juno to not let the Trojans have their way with King Latinus by marriage or besiege Italian borders. Alecto's mission is to wreak havoc on the Trojans and cause their downfall through war. In order to do this, Alecto takes over the body of Queen Amata who clamors for all of the Latin mothers to riot against the Trojans. She disguises herself as Juno's priestess Calybe and appears to Turnus in a dream persuading him to begin the war against the Trojans. Met with a mocking response from Turnus, Alecto abandons persuasion and attacks Turnus with a serpent from her hair. Unsatisfied with her work in igniting the war, Alecto asks Juno if she can provoke more strife by drawing in bordering towns, but Juno replies that she will manage the rest of the war herself: You're roving far too freely, high on the heavens' winds, and the Father, king of steep Olympus, won't allow it. You must give way. Whatever struggle is still to come, I'll manage it myself. (Virgil, Aeneid, trans. Robert Fagles, Book 7, ll.646-649).