The Third War began not with an open invasion, but with tainted grain. The plague transformed the people of Lordaeron into undead. Prince Arthas Menethil pursued those responsible ever farther, purged Stratholme, and claimed the cursed blade Frostmourne in Northrend. His desire to save the kingdom brought him into the service of the Lich King.
Arthas returned home, murdered his father, and destroyed the kingdom from within. The fall of Quel’Thalas enabled the resurrection of Kel’Thuzad, who opened the way for Archimonde. At the same time, the prophet Medivh persuaded Jaina Proudmoore and Warchief Thrall to lead humans and orcs west to Kalimdor.
There, the paths of the Horde, the humans, and the night elves converged. Grom Hellscream drank demon blood once again, but broke free and sacrificed himself to kill Mannoroth, ending the orcs’ bondage. Faced with Archimonde, former enemies formed an alliance.
At Mount Hyjal, the mortal armies bought Malfurion the time he needed. The spirits of nature destroyed Archimonde at Nordrassil. The Legion was defeated, but Lordaeron lay in ruins and the Lich King still ruled the Scourge.
Official Blizzard source: Official World of Warcraft source