Gregor Clegane

In A Game of Thrones, Tywin Lannister sends him to raid the Riverlands in retaliation for Catelyn Stark's abduction of Tyrion. Beric Dondarrion is sent to arrest Gregor, but Gregor ambushes and kills him.[5] He and his men continue raiding the Riverlands throughout A Clash of Kings. When Tyrion Lannister demands a trial by combat in A Storm of Swords, the accuser Cersei Lannister chooses Gregor as her champion. Elia's brother Oberyn Martell volunteers as Tyrion's champion to expose Gregor as a murderer. Oberyn wounds Gregor with a poisoned spear, but the Mountain is ultimately victorious, admitting to murdering Elia and Aegon before crushing Oberyn's skull. Gregor succumbs to the poison and his body is given to Qyburn for experimentation. Later Gregor's skull is sent to Doran Martell and the Sand Snakes who question if it is really Gregor.

Following Cersei's walk of atonement, she gains a bodyguard named Ser Robert Strong who is of similar enormous stature as Gregor, and whose body is completely covered in armor. He is a member of the Kingsguard. Several characters have made comments about his unnatural behaviour and have theorized about who he could be, however he is currently not confirmed to be Ser Gregor.

Tywin Lannister sends Ser Gregor to raid the Riverlands. Beric Dondarrion is sent to arrest Gregor. When war breaks out, Gregor is given command of Tywin's vanguard and left flank and leads his men through intimidation.

Ser Gregor is left to command Harrenhal in Tywin's absence and to find and destroy "The Brotherhood Without Banners", invoking the escape of Arya, Gendry and Hot Pie from Harrenhal. He later abandons Harrenhal after slaughtering the prisoners and is defeated by Edmure Tully at the Stone Mill, but manages to escape back to the Westerlands. Robb chides his uncle, having planned to draw the Mountain into a trap of his own making to be captured or killed.

Ser Gregor is chosen as Cersei's champion for Tyrion's trial by combat, and fights Oberyn Martell, Tyrion's champion who wants to kill Gregor as revenge for the murder of his sister, Elia Martell. Oberyn inflicts several serious injuries on Clegane with his poisoned spear, but Gregor eventually kills Oberyn by crushing his skull, while admitting that he did rape Elia, killed her children and enjoyed it, before collapsing from his own injuries. It is later revealed that The Mountain has been poisoned with manticore venom, a poison with which Oberyn had laced his weapon, and that he is slowly dying. Cersei enlists ex-maester Qyburn to save him, though Qyburn claims that the procedure will "change" Clegane.

The procedure is seemingly a success as Ser Gregor returns to duty, now as a kingsguard, presenting himself as Cersei returns from her walk of shame. Though Qyburn's treatment has changed his physical appearance and behavior, and seemingly made him completely mute, Qyburn explains that Ser Gregor - not naming him as such - has sworn a vow of silence and vowed to serve as Cersei's personal bodyguard until all her enemies are destroyed and evil has been driven from the realm. He picks her up and carries her into Qyburn's care, her demeanor visibly lifted.

Ser Gregor continues to act as Cersei's bodyguard to intimidate all those who may bother or mock her. He also rips the head off of a Faith Militant with his bare hands when they try to take Cersei into their custody. After Cersei destroys the Great Sept of Baelor and retakes power, she has Gregor torture Septa Unella in revenge for torturing her during her time in prison.

Ser Gregor is still with Cersei as a member of the Queensguard. He is confronted by his brother Sandor "The Hound" Clegane in the season finale, where Sandor conveys an ominous message that he (Gregor) already knows who is coming for him and that he has always known.

Ser Gregor's face after going through the procedure Qyburn used to save him after the fight against Oberyn Martell

Ser Gregor is present when Daenerys Targaryen arrives at King's Landing with all her forces, demanding the release of her advisor Missandei and Cersei's surrender. A defiant Cersei has Ser Gregor decapitate Missandei. Ser Gregor is killed in the penultimate episode when Cersei attempts to flee the Red Keep, meeting Gregor's vengeful brother on their way down. Sandor quickly dispatches her guards and Gregor slays Qyburn as he insists upon obeying him and protecting the queen, instead engaging in a sword duel with Sandor (whose presence seems to break his spell of loyalty). After stubbornly fighting his brother by any means available, and even penetrating his bared face with a dagger all the way through his skull (suggesting that Gregor is, in fact, dead and reanimated by magical means), Sandor finally tackles him into an unsteady wall as the Red Keep crumbles around them. The effort is finally successful, and both brothers fall out of the stairwell into a raging inferno below. Ironically, Gregor's ward, Queen Cersei, dies at the same time below them as even the crypts beneath the keep collapse, fulfilling (in a narrow, somewhat ironic sense) Qyburn's promise that the knight would not rest until her enemies had been defeated and "evil driven from the realm".