In Season 5, The Mountain was reintroduced, this time as the Westerosi answer to Frankenstein's monster. He can’t talk and never takes his helmet and armor off, but the small amount of skin that is visible through his helmet is purple and grey, which makes him look more like a corpse than anything. He’s completely loyal to Cersei and acts like her personal bodyguard. The most action we’ve seen from The Mountain since his transformation is when Cersei unleashed the monster on Septa Unella and allowed him to inflict whatever kind of torture he wanted (a penchant for sadistic torture also happens to be the only recognizable trait left from the original Mountain). While fans haven’t seen The Mountain fight since he became a zombie-like servant, it’s clear that he lost any hope of being anything other than a killing machine. If (and realistically when) The Mountain is unleashed on his own brother, The Hound is going to have to bring everything he’s got to beat his monster of a brother.