He's more powerful than any of the four horsemen, having been raised as essentially the most powerful death knight to ever have lived (except he's a ranger).
If you're utterly ignorant of the lore, it explains why you're confused.
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He is basically a more powerful four horsemen, which were 40 man raid bosses in Naxx. Power-wise he should be able to wipe the floor with entire raids single-handedly. He's probably not quite as powerful as Arthas, but close.