The guy who plays "The Mountain" on "Game of Thrones" says he MAJORLY "held back" when he sparred Conor McGregor back in 2015 ... because he coulda Prince Oberyn'd the guy for real.
You remember the video ... 6'9'', 400-pound Icelandic strongman Hafthor Julius Bjornsson toyed with the 5'9" UFC star in a training sesh in Ireland before Conor's big fight against Jose Aldo.
Hafthor talked about the throwdown with RT Sport and said he went easy because, "If I would push all my power into him, I would crush him. Crush him."
"I had to hold myself back because I have too much power for a man his size."
R.I.P. Oberyn's face.