The Mountain is responsible for not only burning the face of his brother, The Hound, but also murdering women and children.
Up until now, much of the damage the character did was off-screen, so casual fans may not recognize him.
Another reason he remains slightly mysterious is because the character has been played by three different actors.
Conan Stevens played him in season 1.
Then in season 2, the role was recast with Ian Whyte.
"Game of Thrones" does occasionally replace minor characters, so recasting The Mountain wasn't too out of the ordinary.
But The Mountain will now be a main focus of the series' next episode (it's titled "The Mountain and the Viper"), so the show searched for another man to fill the role full-time.
Enter Hafþór Júlíus Björnsson, the world's second strongest man.
During a recent Reddit AMA (Ask Me Anything), the 6'9, 419 lbs. weightlifter discussed how he got the part as Westeros' most intimidating force.
"They (HBO) contacted me and called me in for an audition," Björnsson said in the AMA. "During the audition I lifted the guy up (well he asked me to) and they were really impressed."
The massive Icelandic powerlifter-turned-actor added he consumes about 10,000 calories a day and was even recruited to play for NFL's Indianapolis Colts, but ultimately had to decline.
"I had to choose between being a strong man competitor as well as an actor - and the NFL," said Björnsson. "Right now my head and my goal is becoming the Worlds Strongest Man!"
The next episode of "Game of Thrones" will air Sunday, June 1st on HBO.
Watch a preview for the episode featuring Björnsson below: