I pretty much have the same issues and compliments as Gisk, with one additional question. You said due to Dark Arts, he appeared differently to everyone. Is he able to change how he looks?

As for a better weakness, maybe a weak immune system/resistance to poison? All that messing with the human body can't be healthy. Maybe a less subjective effect for when he looks at himself. Like, The Dark Art backfires, and he sees everyone else how everyone else sees him.