Ner'zhul, once thought dead after the destruction of Draenor, instead became a being of both the supernatural and accursed, becoming The Lich King and started to plant the seeds of his and the Burning Legions return, creating a plague of undeath that spread through the Kingdom of Lordaeron, raising those killed as undead under his eternal service.
Princess Calia Menethil, Paladin of the Silver Hand and daughter to King Terenas Menethil ll, fell to corruption after the destruction of her home and her belief in protecting her people at all cost. Taking up the cursed blade Frostmourne, she took revenge against those who had spread the plague. But the blade had corrupted her further, driving her into the service of the Lich King itself. Calia murdered Terenas, defiled the Sunwell of Quel'thalas, and summoned Archimonde, a demon lord of the Burning Legion and left hand of Sargeras who would then go on to destroy Dalaran, the city of Magi.
The survivors of each travesty fled across the sea to the mostly unexplored continent of Kalimdor - the Alliance led by Jaden Proudmoore, and the Horde by Warchief Thrall.