Actually
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Arthas isn't in the Battle of Mount Hyjal. He makes an appearance while talking to Illidian previously, and convinces him to steal the Skull of Guldan, and destroys the Dreadlord overseeing the Invasion of Kalimdor, and turns Half Demon. After that, he doesn't make an appearance until the Frozen Throne.
[13:27:43] [Sabator] peter would be the worst batman ever though. "turn on the bat-signal" "right!" *turns on huge foglight, beams an image of striped pantsu into the sky*