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