From Diablo Wiki
Hammermash, "Brutal Hellspawn Shocker", is a Superunique Mallet Lord that guards the first Hell Rift. It delivers massive blows with a throw of its four arms that easily kill heroes at higher difficulty levels. It uses the Illusionist and Arcane Enchanted Boss modifiers. The Superunique Mallet Lord guarding the second Hell Rift is called Sledge.