Gregor Clegane
The Mountain That Rides: Sandor's monstrous elder brother, near eight feet of murderous rage, Tywin Lannister's favorite instrument of terror.
A soul born at Clegane's Keep, sworn to House Clegane.
The fuller tale
Season One
Ser Gregor Clegane, called the Mountain, enters the Hand's tourney a favorite and leaves it in a rage — unhorsed by Ser Loras Tyrell, who has the sense to ride a mare in heat against Gregor's stallion. He beheads his own horse on the spot and comes for Loras on foot with a longsword, and it is his younger brother Sandor who steps between them and stops a killing in front of the whole city. Gregor walks off without a word, which is somehow worse than if he had said something. His reputation as the worst man in Westeros arrives well before he does anything else to earn it.
Season Four
Named Cersei's champion for Tyrion's trial by combat against Prince Oberyn of Dorne, and it should be no contest — a mountain of a man against one lithe Dornishman with a spear. Oberyn is faster and lands blow after blow, and makes the mistake of stopping to make Gregor confess to raping and murdering his sister Elia and her children instead of finishing him. Gregor answers by crushing Oberyn's skull with his bare hands in front of the entire court, but not before Oberyn's poisoned spear has already opened him up. He is carried off dying, and disappears into Qyburn's care to be worked on by methods nobody in the Red Keep wants described to them.
Season Six
He returns as something else entirely — a hulking, silent shape in full plate who never removes his helm and never speaks, standing at Cersei's shoulder wherever she goes. When the High Sparrow sends a small sparrow to summon her to the Sept, the thing that was Gregor Clegane picks the man up and throws him out of a tower window without breaking stride. Whatever Qyburn did to him, it left a bodyguard who does not tire, does not bleed the way a man should, and does not need persuading to do anything Cersei asks of him.
Season Seven
Qyburn presents him to the small council as Ser Robert Strong, freshly sworn to the Kingsguard, and nobody at that table asks the obvious question twice. At the Dragonpit parley he stands over Cersei in full armor while his brother Sandor stares him down across the yard, close enough to speak to him, and gets nothing back — not recognition, not anger, nothing a living man would give another. Beric Dondarrion holds a flaming sword near him to test something, and whatever it proves, it is not comforting to anyone who remembers what Gregor used to be.
Season Eight
The Battle of King's Landing finally gives Sandor the fight he has wanted since the first time he ever looked at his brother's face — armies and dragons forgotten the moment he spots him in the burning city. He puts sword after sword into Gregor and gets nothing back for it; the thing wearing his brother's face barely registers the wounds, and Sandor accepts a plain fact he has spent seasons dodging — the Mountain other men could hurt died a long time ago. Their fight carries both of them up through a collapsing tower and out into the fire raging beneath the Red Keep, and neither one comes back down.
How it ends for them
On the screen. Ended with his brother in the ruin of the Red Keep (S8·E5)
The Mountain — or the silent thing Qyburn made of him — met his brother on the stair as the Red Keep came down. Blades meant nothing to him anymore; the long fall into wildfire settled what steel couldn't.
How he ends depends on which death you count. In the books he is officially dead already: Oberyn Martell's manticore-envenomed spear killed him by inches in the black cells, his screams administrated by Qyburn, his outsized skull delivered to Dorne to settle a princess's ghost — and the books maintain, with a straight face no reader believes, that the wordless giant serving Cersei thereafter is someone else entirely, his true state a question the published novels have deliberately never answered. The show closes the account in person. As the Red Keep falls, the thing that was Gregor abandons Cersei and Qyburn on the great stair at the sight of his brother — swatting his own maker dead against the stones when Qyburn objects — and meets Sandor in the duel four decades of cruelty had scheduled. He cannot be killed by anything Sandor does: stabbed through the body, blinded in one eye, he simply continues, thumbs seeking his brother's skull in an echo of Oberyn's ending. Sandor's last strength settles it the only available way — a charge that carries them both through the ruined wall and down together into the wildfire-laced inferno consuming the keep. The Mountain dies as he lived, unkillable by rules and killed by his brother's refusal to obey any; the fire that made the Hound unmakes them both, and House Clegane's ledger, opened over a wooden toy, closes in flame.
Their chapters, book by book
A Game of Thrones 1 chapter that name them
A Storm of Swords 1 chapter that name them
Where the page and the screen part
The tellings differ mainly in how much dying he does. The books chronicle the poisoning in luxuriant, screaming detail — weeks of Qyburn-supervised agony filling the Red Keep, a skull duly sent to Dorne as diplomacy, and a strict official fiction thereafter: Gregor Clegane is dead, and the enormous Ser Robert Strong is an unrelated knight of whom no one is permitted to ask questions, his helm never lifted on the page even once. The show, impatient with the pretense, keeps his name, shows his changed face, and gives the revenant seasons of visible service — Unella's warden, the walk's shadow, the head-crushing of rebellious smallfolk — where book Strong has yet to do anything but stand and terrify. The books also keep his pre-poisoning resume longer: the second Harrenhal occupation, the sack of Saltpans blamed on his dead face's helm, and the full horror of his household's attrition arrive there as testimony the show compresses or cuts. Both versions agree on the essential accounting: Elia, the babe, the brazier, and the bill unpaid.
Their journey
Where the two tellings send them to the same place the road runs whole; where they part, the screen keeps the left and the books the right.