Arthas Menethil: Former prince of Lordaeron and paladin of the Holy Light. And then, traitor to his kingdom and humanity, first death knight of the undead plague.

Ner'zul: Ancient orc shaman, very powerful, with great seer and necromantic powers. And later, turned into a specter trapped in the crystal of the Frozen Throne, and being the leader and creator of the plague.

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Their merged souls have formed this new being. To the master of death, to the dark lord of the dead, to the embodiment of cold... To the Lich King.