One of the most unique bad guys in fiction, the Mountain is a minor villain in the Game of Thrones series. During Robert's Rebellion, he was with his lord Tywin Lannister as they sacked King's Landing. He was the one who murdered Elia Martell and the Targaryen children, ironically having been knighted in the past by Prince Rhaegar himself. As a child, he stuck his brother the Hound's face in a firepit for playing with one of his toys. And as a man, he's a brute and nearly unstoppable.
During the War of the Five Kings, he rides with his own army and destroys much of the Riverlands. Committing many unspeakable acts along the way. He is called back to King's Landing to serve as the Crown's champion in a trial by combat for Tyrion Lannister's crime.
And Oberyn Martell volunteers to fight for Tyrion as it gave him the perfect chance to avenge his family. One of the best fights in the whole series. Oberyn gives him a mortal wound, but the Mountain manages to kill the Red Viper before his own death.
Then we find out that Qyburn "healed" him into a zombie. And he becomes part of King Tommen's Kingsguard.
Though he really just protects Queen Cersei. Which was her only real defense against the Faith Militant. None of them dare try to arrest her now that he's there. He proves to be almost immune to wounds or death.
And he stays around when Cersei anoints herself as the ruling Queen. He is the one who slays Missandei on Cersei's command.
But he leaves his Queen to fight his brother in one last duel that would claim both of their lives. I didn't really like Cleganebowl. It was boring, dull, and most of all, pointless. But at least they had a way to kill off a nearly invincible character. I had assumed it might have been Drogon to do the deed.