For me, it was Drela from Drela's cottage. Mother of god, he uses the most powerful attacks I've ever seen a wizard use. When I encountered him inside his home where he immediately became hostile, I was like "fair enough. He just seems like your run of the mill wizard". So, doing as I normally do, I charge him and power attack him, when suddenly BAM! He uses a flamecloak spell and kills me in one hit, and my heavy armour rating is 397 at level 24.
So I reload, try again, and he starts using flamecloak and summons a pretty powerful frost atronauch, and I died within minutes of facing him as he used frost spells on me.
That said, I don't think I'll be going back there for quite some time.

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