They used to be Humans and Elves before the Scourge came and poisoned the unlucky ones, turning them into Undead monsters. They sacked the human city of Lordaeron forcing the inhabitents to flee for their lives. During a battle between the Lich King and Illidan Stormrage, a group of the Undead broke free from the powers of the Scourge. Led by Sylvannas Windrunner, they took control of Lordaeron and built a City underneath. They hate the term the Undead, preferring to be known as the Forsaken instead.
Copyright: Blizzard Entertainment