Ner'zhul

From WoW: IncGamers World of Warcraft Wiki

Elder Shaman Teacher of dark magics to Gul'dan. After the Second War and failing to defeat the human opposition on Azeroth on behalf of the Burning Legion, he was kept alive and tortured by Kil'jaeden, and then sent back to Azeroth locked in a magic ice crystal as the immensely powerful Lich King.

The Lich King then continued to increase his power in Northrend, where he was located on his Frozen Throne, and then managed to spread a plague in Lordaeron that made anyone killed by it into an undead, under his control. Arthas tried to conquer this threat, but in the end he succumbed to the Lich King, selling his soul to Frostmourne, and by the end of the Third War he merged with the Lich King to become the New Lich King.