Hammermash, the Brutal Hellspawn Shocker, is a Super unique Mallet Lord found in a Hell Rift in Act IV of Diablo III.
He has Electrified, Arcane Enchanted and Illusionist monster affixes.
Killing him is not required for the Hell Rift quest. He will always spawn in Campaign mode, and either he or Sledge can be a bounty in Adventure Mode.