In the Warcraft lore, Arthas Menethil did die, but his soul was not completely destroyed. Instead, his soul was merged with the soul of Ner'zhul, the orc shaman who became the original Lich King. This fusion of souls resulted in the creation of a new entity known as the Lich King, which was responsible for the creation and control of the undead Scourge.
So while Arthas did physically die, his spirit lived on in a twisted form as the Lich King. Arthas' story is a significant part of the Warcraft lore, particularly in the Warcraft III: Reign of Chaos and Warcraft III: The Frozen Throne video games, as well as the World of Warcraft MMORPG.
