From the history of World of Warcraft - new "Wrath of the Lich King" trailer.
Quote:
...Arthas and his Scourge invaded Quel'Thalas and laid siege to the elves' crumbling defenses. Sylvanas Windrunner, the Ranger-General of Silvermoon, put up a valiant fight, but Arthas eventually eradicated the high elf army and battled through to the Sunwell. In a cruel gesture of his dominance, he even raised Sylvanas' defeated body as a banshee, cursed to endless undeath in the service of Quel'Thalas' conqueror...