The Reaper is a Dawnguard (DLC) exclusive, in the soul cairn you can find reaper gem fragments which can summon The Reaper. You have to find 3 of them but there are more than 3, you then need to place all 3 of the fragments on his altar and you can fight him and a few bonemen. He is referenced throughout the soul cairn as NPCs can be heard saying “Shh, he’s watching us.”, he also drops a daedra heart.
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