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Vargul are a type of undead vrykul. They have been raised into a lesser form of undeath because they were not found worthy by the Lich King.
They are the losers of the challenge to "ascend" at Jotunheim. The winners are named ymirjar. They are changed by val'kyr after their failure.
Unlike other undead, they appear to retain some level of intelligence, and consequently many resent their fate. Thus the major difference between vargul and lesser undead minions is that vargul still possess enough of their minds to despise being used for labor. Never will a vargul be pleased to be working aside skeletons or ghouls.