Oberyn Martell
The Red Viper: Doran's hot-blooded younger brother — poisoner, duelist, scholar and lover — who has never forgiven the Lannisters for the murder of his sister Elia.
A soul of House Martell, born at Sunspear.
The fuller tale
Season Four
Arrives in King's Landing for Joffrey's wedding with Ellaria on his arm and an old grievance he has never let go of: his sister Elia was raped and murdered during the Sack of King's Landing, and the man who did it, Gregor Clegane, still serves the family Oberyn has come to visit. He needles Tywin and Cersei about it without much subtlety, samples the city's brothels and poisons with equal expertise, and takes an interest in Tyrion's murder trial mostly because the man in the dock is a Lannister he does not hate. When Tyrion demands a trial by combat, Oberyn volunteers as his champion — not out of loyalty, but because it is finally a legal way to put a spear through Gregor Clegane. He fights brilliantly, gets his confession, and then lingers over it, demanding Gregor say Elia's name clearly instead of finishing him — and it costs him his skull in front of the entire court.
Season Five
Dorne mourns Oberyn; Ellaria wants war on children; Doran wants proof and patience.
How it ends for them
On the screen. Killed in single combat (S4·E8)
The Red Viper danced circles around the Mountain in Tyrion's trial by combat, demanding a confession for his sister Elia. He got the confession — with Gregor's thumbs in his eyes and his skull bursting in the Mountain's fists.
On the page. Killed in trial by combat (ASOS, ch. 71)
The Red Viper fought the Mountain for Tyrion's life and his sister's memory, poison on his spear and 'say her name' on his lips. He had his confession — Gregor roared it while he crushed the prince's skull with his bare hands.
He dies in the duel he was winning, of the victory he actually wanted. Champion for Tyrion against Gregor Clegane, Oberyn fights the trial by combat entirely on his own terms — a spear against a mountain, wine in one hand in the books' telling, poetry of footwork in the show's — and puts the great knight down, poisoned steel through the armor's seams. Then, with the verdict his for the taking, he declines it: circling the fallen giant, demanding the confession, the name, the order-giver, refusing to end the collection before the debt is read out in full. The Mountain's hand finds his ankle; the rest is the story's most infamous minute — Oberyn borne down, thumbs in his eyes, his skull burst in Gregor's fists even as the dying monster roars out the confession at last. Both tellings agree on every essential, including the bitter accounting: the Viper got his names said, Tyrion got a death sentence, and Gregor got Oberyn's manticore-venom parting gift, which the books make the seed of something worse than death. Dorne's answer to the news fills the rest of both sagas.
Their chapters, book by book
A Storm of Swords 2 chapters that name them
A Feast for Crows 1 chapter that name them
Where the page and the screen part
Book and show Oberyn are unusually aligned in character and diverge in aftermath: the books make him the trigger of the entire Dornish master-plot — his death radicalizes Arianne's generation, forces Doran to reveal the Targaryen pact, and leaves eight Sand Snakes (not three) for the prince to disarm — while the show routes his legacy into Ellaria's vengeance coup, a plot his book paramour explicitly argues against. Book texture the show trims: the Yronwood duel that named him, the maester's links, a stint founding a sellsword company, and his correspondence with the lamed Willas Tyrell — the books' sly proof that the Viper's venom was always selective. The brothel diplomacy and the cell-side account of Elia are both tellings' shared jewels, near verbatim.
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.