Ser Gregor Clegane is the head of House Clegane, minor vassals of House Lannister. Ser Gregor is a notoriously fearsome warrior with a tendency toward extreme violence. During the War of the Five Kings he raided the Riverlands on orders from his liege lord Tywin Lannister. Ser Gregor served briefly as castellan of Harrenhal before withdrawing his troops.
Ser Gregor Clegane is the head of House Clegane, a knightly house from the Westerlands, and the elder brother of Sandor Clegane. When Sandor and Gregor were children Gregor held his brother's face in a fire for using his toys without permission, horrifically scarring him. Gregor is a landed knight rather than a full lord. Gregor is freakishly huge, said to be the largest man in Westeros, almost eight feet in height and heavily muscled. For this reason he is called "The Mountain That Rides" or, simply, "The Mountain". He is feared throughout Westeros with a reputation for ferocity and anger. He is a loyal retainer and servant of House Lannister, particularly to Lord Tywin.
Numerous servants have gone missing at the Clegane keep over the years, apparently killed by Gregor for minor slights in one of his infamous rages, and their deaths then hushed up. Gregor's own father and sister died under dubious circumstances as well, and Sandor half-suspects that Gregor may have killed them as well. Their sister died when Sandor was too young to remember her well, and their father died years later in what was said to be a "hunting accident". At the behest of Tywin Lannister, Prince Rhaegar Targaryen himself anointed Gregor as a knight, a great honor, even though Gregor was an infamously dishonorable man.
During Robert's Rebellion, one year after Prince Rhaegar knighted him, Gregor participated in the Sack of King's Landing. He raped and murdered Rhaegar's wife, Princess Elia Targaryen, after killing their baby son, Prince Aegon, by bashing his head against the wall, while Ser Amory Lorch killed their daughter, Princess Rhaenys.
Despite pleas by Ned Stark, neither Gregor nor Lord Tywin were punished by King Robert for the murders of the royal family, leading to a rift in Ned and Robert's friendship.
Ser Gregor takes part in the tournament to celebrate Eddard Stark's appointment as Hand of the King. He kills Ser Hugh of the Vale in a joust, horrifying the crowd. Littlefinger tells Sansa Stark that Gregor burned his brother Sandor's face over a naked flame when they were boys because Sandor borrowed one of his toys without asking. He tells her that few people know the story and not to let Sandor Clegane know that she does. When Gregor faces Ser Loras Tyrell, he loses the joust because Loras chose to ride a mare that was in heat to distract Gregor's stallion. Flying into a rage, Gregor demands his sword, then decapitates his own stallion with a single swing of his sword, before the entire horrified crowd. Ignoring his surroundings, he proceeds to knock the surprised Loras off his horse, and is on the verge of dishonorably killing him, when his brother Sandor intervenes, fighting him off to protect Loras. Gregor only stops when King Robert himself shouts a demand to end his folly, at which the fuming Ser Gregor relents and leaves.
Catelyn Stark takes Tyrion Lannister into custody on suspicion of the attempted assassination of her son Bran. In retaliation Gregor is dispatched to raid the Riverlands, her homeland. Refugees from the Riverlands attend court in King's Landing and report that a huge knight who could take the head of a horse in one blow led the attack and had sacked their villages. Littlefinger says "Remind you of someone?" Eddard Stark sends Ser Beric Dondarrion with a hundred men to bring Gregor to justice. Eddard declares him a false knight and strips him of all ranks and titles, lands and holdings, and sentences him to death. The ruling is made null by Eddard's arrest for treason and subsequent beheading.
The host led by Lord Beric clashes against Ser Gregor at the Mummer's Ford. Ser Gregor is victorious, and Lord Beric is reported dead.
However, when Robb Stark's victory against his son forces Lord Tywin to withdraw his forces to Harrenhal, he orders Ser Gregor to take five hundred men and "set the Riverlands on fire" from the Red Fork to Gods Eye.
Gregor continues to raid the villages of the Riverlands from the Lannister army's base at Harrenhal. His men torture the prisoners they take for information about the Brotherhood and hidden valuables. He selects the prisoners for torture personally. Ser Amory Lorch adds a group of captured Night's Watch recruits to the captives. Lord Tywin Lannister arrives during the torture and asks Gregor what is going on. Gregor says that they were not expecting Tywin for another day. Tywin asks why the prisoners are not in their cells. Gregor says that their cells are overflowing. One of Lorch's men interjects that the prisoners will not be there long. Tywin asks if they are so well manned that they can afford to discard able and skilled prisoners. He asks a prisoner if he has a trade and learns that he is a smith. Tywin orders the prisoners put to work, sparing the young man.
Following the assassination of Ser Amory just outside Lord Tywin's chamber Ser Gregor is tasked with finding the culprit. Tywin fears that he was the intended target and has Ser Gregor hang dozens of their own men. Ser Gregor suggests that an infiltrator from the Brotherhood Without Banners was responsible and Lord Tywin dispatches him to pressure the smallfolk of the Riverlands into giving up the members of the Brotherhood. Tywin marches back to the Westerlands and leaves Gregor to serve as castellan at Harrenhal.
Robb Stark and his army get to Harrenhal, finding that Gregor Clegane has left the castle with his men. To their horror, they discover that Ser Gregor has executed the remaining 200 Northmen and Rivermen prisoners at Harrenhal, leaving their bodies to rot.
Ser Gregor is later expelled from Stone Mill by a force led by Lord Edmure Tully, forcing him to flee south to Casterly Rock.
Oberyn Martell discusses with Tyrion Lannister how the last time he was in King's Landing, it was for the wedding of his sister Elia to Rhaegar Targaryen. He also states that during the Sack of King's Landing, his sister was raped and murdered by Gregor. Oberyn believes that Tyrion's father, Tywin, was the one to give the order to the Mountain.
Polliver tells Gregor's brother when meeting in a tavern further north, that whilst loyal to the Mountain and the Lannisters, Polliver wasn't in agreement with Gregor's torturous methods because they were repetitive and boring.
When Tyrion Lannister demands a trial by combat, Gregor is appointed the Crown's champion by Queen Cersei Lannister. Tyrion first asks his brother Jaime to fight for him, but he refuses, saying that he simply cannot fight like he used to. Tyrion then asks his friend Bronn to be his champion, but he too refuses, arguing that the Mountain is an incredibly dangerous fighter, one that would cost him his life if he makes a single mistake. Finally, Prince Oberyn Martell approaches Tyrion and volunteers to be his champion, as he wants to avenge his sister.
Gregor fights Oberyn at Tyrion's trial by combat, wielding heavy plated armor and a large broad sword with one arm. Despite his immense size and strength, he is no match for Oberyn's swift and agile fighting style. Oberyn manages to knock Gregor's helmet off early into the duel and proceeds to slowly cut The Mountain down, all the while taunting him and demanding him to admit to the rape and murder of Elia Martell and her children, chanting "you raped her; you murdered her; you killed her children". Oberyn eventually severs Clegane's hamstring, causing him to fall to his knees. Oberyn proceeds by jumping and thrusting his spear into Gregor's chest, severely injuring the gigantic man and seemingly sealing his defeat. Thinking his opponent is fully incapacitated, Oberyn removes his spear from Gregor's chest and circles him, refusing to initiate the killing blow until Gregor has admitted to Elia's death and revealed who orchestrated it, directing his gaze accusingly at Tywin. However, in his hubris Oberyn is unexpectedly caught off guard when Clegane trips and pulls him to the ground, grasping him around the neck and slamming his fist squarely into his face, instantly knocking out many of Oberyn's teeth. Clegane then begins crushing Oberyn's eyeballs with his thumbs while sadistically admitting to Elia's rape and murder, finishing the duel by brutally smashing Oberyn's skull in with his bare hands. As Gregor collapses to the ground from the wounds he sustained, Ellaria and Tyrion look on in horror as Tywin announces that, according the rules of trial by combat, Oberyn's death has officially sealed the fate of Tyrion Lannister and sentences him to death.
Secretly, Oberyn had his spears coated with Manticore venom, one of the deadliest poisons in the known world. Gregor is left in agony and ultimately loses consciousness. The venom causes his wounds to start putrefying, emitting an overpowering stench, and with dead flesh sloughing off. Even Grand Maester Pycelle regretfully tells Queen Cersei that Ser Gregor is beyond any hope of healing. Qyburn, however, an expelled former maester with extensive medical knowledge gained from "repugnant" and "unnatural" experimentation on living people, suggests that he may be able to save him. Cersei dismisses Pycelle and gives Qyburn permission to do anything he can to save Ser Gregor, though he says that his unorthodox methods will "change" him...
A moribund Gregor Clegane is being experimented on by Qyburn at his laboratory in the dungeons below the Red Keep. Queen Cersei visits Qyburn and asks him about the state of his work, while glancing at Gregor's covered body, and Qyburn assures her it is going very well, though there is still a way to go. Then, Gregor suddenly spasms, and Qyburn gently shushes him and tells him to go easy. During Cersei's imprisonment by the Faith, Qyburn reminds her that "the work continues.
After Cersei's walk of atonement, Qyburn covers her up and is pleased to introduce the "newest member" of the Kingsguard. The Mountain stands as tall as ever, fully covered in Kingsguard plate armor. Only his reddened eyes, surrounded by pale blueish flesh, are visible through his helmet. With Cersei's feet injured and bleeding, he is instructed to carry her. As he does so, Qyburn informs Cersei that he has taken a holy vow of silence to last until all of Tommen's enemies are dead and the evil has been driven from the world.
Gregor is present standing guard over Cersei when Jaime returns with their daughter's body. Later, Gregor continues to serve Cersei without question. He finds a man who boasts of having flashed his penis at Cersei during her walk and bashes his head against a wall, killing him.
Back at the Red Keep, he accompanies Cersei when she attempts to attend Myrcella's funeral, only to be stopped by a group of Lannister soldiers, who have arrived to prevent her from leaving on King Tommen's orders. Although Gregor intimidates the captain with his size, Cersei acquiesces, and he escorts her back to her room
Gregor follows Cersei and Jaime into Qyburn's laboratory, where the former Maester is trying to win the service of the Little Birds by offering them candied plums from Dorne. Jaime asks what Qyburn has done to Gregor, to which the latter replies with "a number of things". Jaime wonders whether Gregor is able to understand complete sentences, to which the huge knight menacingly turns his head towards Jaime. Qyburn assures that Gregor understands well enough. Jaime suggests sending Ser Gregor to the High Septon to crush him and his Faith Militants. Cersei disagrees, saying they can't let Ser Gregor face them all and that he only needs to face one, implying the coming Trial by Combat.
At the Small Council, Pycelle expresses his disgusts about Qyburn's treatment of Ser Gregor, calling him an abomination. He attempts to suggest that it would be in everyone's best interest to have the "beast" destroyed, but his cut short when he becomes aware of the presence of Cersei, Jaime and Gregor himself. Frightened by the huge man in shining armor, Pycelle cannot keep himself from passing gas. Kevan argues that Cersei and Jaime have no position on the Small Council, but Jaime replies by saying the Lord Commander of the Kingsguard have. Cersei suggests the Small Council get on with urgent matters, as they cannot make them leave. Kevan replies that Cersei and Jaime cannot make them stay either, unless they set Gregor to kill them. As everyone except Cersei and Jaime leave, Pycelle cowers past Ser Gregor, who coldly looks back.
After Cersei got word of Olenna Tyrell's intention to return to High Garden, she confronts her. Ser Gregor Clegane follows in the Queen Mother's wake and silently stands guard behind her
When Lancel and several other members of the Faith Militant enter the Red Keep, Cersei, Qyburn and Ser Gregor arrive to confront them. Lancel tells Cersei that the High Septon wishes to speak with her, but Cersei refuses, and when Lancel does not leave at her command, Gregor stands in his way as the Faith attempt to forceably take Cersei. He remains unmoved when Lancel orders him to stand down. One of the Sparrows attacks Gregor, hitting him in the chest with a weapon, but it becomes lodged in the armour and does not affect him. When the Sparrow tries to attack again, Gregor grabs him by the throat, lifting him into the air and throwing him to the ground. While Lancel and the other Sparrows look on in horror, the towering knight kneels beside him and tears his head off. Lancel and the others thereby stand down.
A massive knight who stands more than seven feet tall, Ser Gregor Clegane is known as "The Mountain That Rides". He has served the Lannisters as a vicious thug along with his brother, Sandor Clegane, but the two bore no love for each other. During the wartime sack of King's Landing, Gregor raped and murdered Ella, wife of Prince Rhaegar, and killed their two young children. Gregor was critically injured and poisoned by Oberyn Martell during Tyrion Lannister's trial by combat. He was treated by Qyburn and now serves Cersei Lannister as her silent Kingsguard.
Gregor is well-known for his brutal and savage nature, remorselessly killing unarmed opponents half his size or leading the torture of helpless villagers. Often referred to as "Tywin Lannister's mad dog", Gregor has an insatiable bloodlust that he releases through acts of extreme violence. He is quick to anger and never shows any fear of acting on it, since no one dares stop him. The only person he has ever shown any loyalty or respect toward is Tywin, otherwise only seeming to act on his own accord, showing even little respect toward King Robert Baratheon after he was told to stop his fit of rage at the tourney. His violent personality had even manifested as a child, quickly becoming known for his strength, size, and brutality. Gregor even burned his own brother's face when they were children just because one of his toys was being played with. Gregor's short fuse and lack of conscience often showed his lack of honor, such as when he tried to murder Loras Tyrell after already losing a joust against him.
As a warrior, Gregor is feared for his imposing size and physical strength. In combat, his strength gives him a huge advantage over almost any opponent, able to overpower his enemies with brute force rather than rely on his skill. His weapon of choice was an enormous broad sword that most men would never be able to swing with even two hands, but which Gregor is strong enough to wield one-handed. He is strong enough to decapitate a horse with a single swing of his sword. Gregor also wears armor larger and thicker than normal knights would be able to wear, providing him almost impenetrable defense against his opponents. However, his brother Sandor Clegane (who is also very large and strong, though not to the same extent as Gregor) was shown to be able to duel him evenly. In addition, Gregor's lack of technical skill allowed Oberyn Martell to overwhelm him in their duel, as Oberyn's unique style of combat relied heavily on maneuverability and he used a spear to stay out of Gregor's reach.
Following being poisoned by Manticore venom, Gregor is experiment on by Qyburn to keep him from death. The process changes Gregor as was expected. Unknown whether to be truly dead or alive, the Mountain becomes a "new" man and now part of Cersei's guard. Other then his soulless eyes and pale flesh visible through his helmet, he can not be seen under his full body armor. Whatever he is now, he is completely in service to Cersei (and Qyburn), and is will be silent until his vows are completed.