If he is the last brother you are killing, ensure that you have all of the other brothers' necroplasm in your inventory and enter the tunnels below. Go in and out of any nearby door until Ahrim the Blighted spawns and kill him. Once defeated, climb the rope back to his crypt and use the Greater necroplasm on his sarcophagus.
With all 6 necroplasm types in your inventory, return to the tunnels and make your way to the center. Be sure to NOT fail the puzzle door as this will negate your ability to combine the necroplasm. If you fail it, the necroplasm will not combine when you loot the chest. You will need to loot as normal and complete another Barrows run to try again.
The Brothers necroplasm includes: