The Reaper is a Dawnguard exclusive, so your first challenge is to get the DLC. Then, in the soul cairn you’ll be able to find reaper gem fragments to summon this boss. You’ll only need three, which need to be placed on his altar. He appears with several bonemen and when defeated, he’ll drop a daedra heart.
Through the area, NPCs will be found whispering ‘Shh, he’s watching us’. Time to put his watching to an end!