You don't get anything special from beating the reaper.
from the wiki
Using the spell Detect Dead or the Shout Aura Whisper will reveal all of the Reaper's minions inside the floor. By standing above one of these minions and attacking, the Reaper and his minions will rise and attack the player, at which point they can be fought and killed without placing or gathering any of the Reaper Gem Fragments. Another way, not requiring the Detect Dead spell, is by walking to the far side of the room from the door. The ghostly smoke effect of the reaper can be seen through the pile of bones he is standing in, about where his head would be. If the Dragonborn goes up to him and tries to select him as if to initiate dialog, he will step forward and his minions will rise.
You can get around having to collect the Reaper gem fragments by scattering a few Fire Runes on the ground near him. This will summon his minions from below and draw him out for the fight. Just move to the back of the room and cast a mine on the large pile of bones.
Testing: Werewolf/Follower Glitch; Armor usable with Mage Armor and hidden +25 armor/pc; Augment Ring of Necromancy