Technically, the Mountain died at the tip of Oberyn Martell's poisonous spear back in season four during the Trial for Combat to determine Tyrion's fate. He is soon brought back to life by Qyburn, and the procedure seemingly makes the Mountain invincible. In season eight, his brother the Hound attempts to kill him by sticking a sword through his chest, eye, and brain with no tangible results. It's not until the Hound yanks them both over the edge of a balcony into the fiery depths below that the Mountain meets his end.