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Its not a matter of Why he did its a matter of howhe did it wasn't his choice, it was the Lich King who chose him long before he was even born apparently to be his champion, so with Mal'Ganis's aid the Lich King was able to draw Arthas to Northrend and to Frostmourne which ended up turning Arthas into what you see today, and it wasn't actually until he fused with the Lich King in WC3:TFT that he became "king of the dead" Hope that answers ur question =D
The Lich King was once Arthas Menethil, formerly a Human. Currently as a Death Knight his type is Undead. So technically he is an Undead Human.
king terenas who was slain by arthas himself
No, Uther was the first Paladin. He was also a mentor to Arthas. Arthas's father was King Terenas Menethil II.
Yes, Arthas Menethil is the Lich King. You can see a cinematic of the Lich Ling by clicking on the plinth of the fountain in Dalaran.
he's the lich king Arthas was the prince of lordaeron and uthers pupil. On his hunt to rid his land of the undead he chased mal ganis. he found a sword called frostmourne and became a death knight. he killed his father and uther and became the lich kings servant. later on he heads to northrend the capital of death and merged with the lich king and became the lich king.
The Lich King is the commander of all the Scourge forces, and it is his will that keeps them under control. Without a Lich King to guide them, they would rampage chaotically around Azeroth, killing as they go and creating new undead. Azeroth would become a dead world, and even Arthas understood he'd need new living beings to continue his Scourge army.
The Lich King is a human by the name of Arthas Menethil.
Technically, Arthas became the Lich King, and currently people can fight the Lich King in Ice Crown Citadel, he is the final boss in this raid.
Arthas Menethil. He is fused with Ner'zhul.
Ilidan was slain by Arthas, the Lich King when the two did battle.
He is what you know now as the Lich King. But he was better known as Arthas in Warcraft I, Warcraft II, Warcraft III, as a holy warrior (paladin )that has fought scourge.