Hafthór Björnsson became famous for playing Ser Gregor Clegane, “The Mountain That Rides,” on season four of HBO’s Game of Thrones—the third actor to play the role (previously played by Conan Stevens in season one and Ian Whyte in season two)—culminating in an epic battle to the death with Prince Oberyn Martell, “The Red Viper.” But while the character he plays on Thrones is an awful beast of a man, it’s what he’s done off the small screen that has truly endeared him to the masses such as breaking 1,000-year-old viking weightlifting records while dominating strongman competitions or just being plain awesome for the kids.

Now Hafthor is taking on one of the UFC’s up and coming superstars, current interim Featherweight division titleholder Conor McGregor. The two sparred together recently, and you can see the video of their session below.