His dramatic transformation occurs after King Joffrey Baratheon's assassination.

Source: https://screenrant.com/game-thrones-mountain-gregor-clegane-reanimation-books-cut/

Now you can find out if the White Walkers have made it across the Wall, if King Joffrey is on a murder spree and whether the Queen Of Dragons has turned any armies into toast recently.

Source: https://www.sundaypost.com/fp/love-janessa-bbc-soundsyou-meet-someone-online-but-it-turns-out-many-others-think-they-have-fallen-for-the-same-person-weve-all-heard-of-internet-scams-but-often-those-used-by-the-scamme/