This supposedly malevolent character has never been seen in the story, and is a myth that book-readers are expecting to be the Night King in some fashion.
But considering the books have not already introduced him, it would be odd to just place him in the story.
Will readers be able to see a brand new antagonist in the coming books?
Meanwhile, it has also been suggested the books will not complete the story of the Hound and the Mountain.
Outlander author Diana Gabaldon gives update on book 9 point of views [INFO]
While the brutal brothers have been battling one another for decades, their fates in the books are completely different to how the show has aired.
The Mountain suffered a horrific blow during his fight with Oberyn Martell – just like in the show – however the book version didn’t survive.
Instead, Maester Qyburn sent the Mountain’s head to the Martells as a form of apology.
The Mountain’s body was then given a new head before being resurrected by the evildoer.