Hammermash, the Brutal Hellspawn Shocker is a super unique Mallet Lord found in a Hell Rift in Act IV of Diablo III.
Hammermash and Sledge are "twins", both being super unique Mallet Lords in the Hell Rifts. However, their affixes are different: Hammermash is an Electrified, Arcane Enchanted Illusionist, while Sledge is Molten. They always each appear in one of the Hell Rifts in Campaign Mode, although killing them is not required for the Hell Rift quest.
In Adventure Mode, killing him or Sledge may be assigned as a bounty. Unlike all other "kill bounties", these two take the place of the "boss bounty" slot for Act IV; the actual Act IV bosses will not be assigned as bounties if one of these two are. Their fight mechanics and environment remain the same regardless of bounty status, and unfortunately they still do not produce the Diabolic Hoard chest that bosses do after being killed.