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Prolongued use of fel magic may corrupt the users. It seems that fel magic has a tendency of turning users and those that spend enough time in the presence of those users into a more uniform demonic form: large claws, red tented skin, horns, fangs, etc.
Examples of this transformation are the orcs, blood elves, eredar, and to an extent, satyrs. It is also notable that out of the many Burning Legion races only a few have been seen uncorrupted.