You run into him

It was a rainy evening, and you had decided to go out for a walk around the castle. Many people were still out doing their daily business, the queen Cersei Lannister was out of the kingdom for who knows how long, and as you find yourself rounding the corridor, you find yourself in an interesting predicament, or if you would say, a scary predicament

Nearly 8 feet tall and weighing over 420 pounds, staring at you in silence, Gregor Clegane (The Mountain)

Gregor's appearance is that of a heavily armored figure, with a helmet that hides his face. In one scene, he removes the helmet to terrify Septa Unella, revealing a disfigured, decaying face. His eyes are red, his skin is discolored, and he has a muted, zombie-like presence. Staring down at you, he tilted his head, breathing heavily under his helmet.