Oberyn Martell

"Tell your father I'm here - and tell him the Lannisters aren't the only ones who pay their debts."

- Prince Oberyn Martell to Tyrion Lannister

Oberyn Martell is a major character in the fourth season. He is portrayed by Pedro Pascal and first appears in "Two Swords". He's a member of House Martell, the ruling family of Dorne.

Prince Oberyn, known as the Red Viper of Dorne, is the younger brother of Doran Martell, the Prince of Dorne, and the late Elia Martell, wife of Rhaegar Targaryen, the late Prince of Dragonstone and heir to the Iron Throne.

Background
Prince Oberyn is the younger brother of Prince Doran Martell and Princess Elia Martell; like most members of House Martell, he is styled "Prince" instead of "Lord". Oberyn is known for being an accomplished warrior, but Tyrion implies that he is just as infamous for his sexual appetite. Oberyn's official paramour is Ellaria Sand. He has eight bastard daughters, the last four born to Ellaria Sand. He named the fifth one, Elia Sand, after his late sister.

Prince Oberyn trained at the Citadel in his youth, in order to become a Maester. Although he ultimately abandoned his studies, he became well versed in the use of poisons in the process.

Oberyn also travelled to Essos in his youth and saw the Unsulied in Astapor, who he states are impressive on the battlefield, but less in the bedroom. He lived there for a total of five years.

Season 4
Tyrion Lannister meets the lords of Dorne outside King's Landing, only to find out that Oberyn will be representing House Martell at the king's wedding, and not his brother, reigning Prince Doran. The Dornish lords tell Tyrion that Oberyn in fact arrived in the city before dawn. It doesn't take Tyrion long to realize that Oberyn is in one of Littlefinger's brothels. Oberyn assists his paramour Ellaria in choosing a whore for her own enjoyment before selecting Olyver to satisfy himself. Oberyn's fun is cut short when he hears Lannister men singing the Rains of Castamere, which sends Oberyn into a quiet rage. Before one of them can draw his sword, Oberyn drives his dagger through the man's wrist, though he assures the man's compatriot that he can be saved, if his injury is treated immediately. Tyrion arrives, and interrupts the fight. Leaving the brothel, Tyrion asks Oberyn why he came to the capital. Oberyn, much to Tyrion's discomfort, relates the story of how Gregor Clegane, Tywin's bannerman, raped and murdered Elia Martell and slaughtered her children. He then warns Tyrion that the Lannisters aren't the only ones who paid their debts.

Later, Oberyn attends Joffrey's wedding, where he meets Tywin Lannister and Cersei Lannister. He introduces Ellaria to them; however, after they clearly express their distaste for bastards, Oberyn discreetly argues with the two Lannisters over the cultural differences between the Dornish and the rest of Westeros. Oberyn also gives Loras Tyrell a very flirtatious look, which leaves Loras blushing.

In the days following Joffrey's death, Oberyn and Ellaria visit the brothel again. Oberyn grows angry with Olyver when the prostitute rejects Ellaria's advances (as he doesn't have the taste with women), but Ellaria calms him down. Oberyn then discusses his approach to sexuality (not restricting himself to intimacy with one sex and having as much sex as possible before old age) with Olyver. The small orgy is broken up by the sudden arrival of Tywin. After briefly discussing Oberyn's experience studying poisons at the Citadel, the Hand asks Oberyn to be the third judge at Tyrion's trial. Oberyn initially refuses, confirming that he blames Tywin for Elia's death. However, he reconsiders when Tywin offers Oberyn in particular and Dorne in general a seat on the Small Council. Tywin explains that he wants to reunify the realms because he knows that one day, Daenerys Targaryen will return... and the last time dragons assaulted Westeros, only Dorne stood against them. Oberyn is pleased that Tywin has, in a roundabout fashion, admitted that he needs Dorne. He is also placated by Tywin's promise to arrange for him to confront Gregor Clegane privately.

Following the coronation of Tommen Baratheon, Oberyn takes some time to write a bit of poetry in the gardens to his fifth daughter Elia, named after his dead sister. Cersei visits him and asks for a word. They discuss their children and Oberyn reveals he has eight bastard daughters. Cersei asks after her daughter Myrcella, and Oberyn tells her that she is doing well and last he had seen her, she was playing with two of his daughters in the Water Gardens back in Dorne. Cersei asks Oberyn to deliver a gift to Myrcella, a ship, and then departs, leaving the prince trying to decide if he is to sympathize with a Lannister, though he promises to have it sent to Sunspear.

Oberyn later takes up his seat on the Small Council. In typical fashion, he complains loudly about the early meeting, pretends to be more concerned about the fancy title he will receive than matters of state, and refuses to rise when Lord Tywin enters the room. In spite of this, Tywin seems more interested in his opinion than that of Mace Tyrell, the new Master of Ships. Oberyn also warns the Small Council not to discount the danger that the Unsullied pose on the battlefield.

After the meeting, Oberyn approaches Varys in the Throne Room, with the Dornishman surprising him with his knowledge of Essos. Oberyn shares that he spent several years in Essos, and spots that Varys is originally from Lys, unsettling Varys who believes he has no noticeable accent. The men exchange uneasy banter, and Varys makes it clear that unlike Oberyn with his thirst for pleasures, his interest lies only with the Iron Throne.

Later on, Tyrion's trial begins. King Tommen recuses himself from the trial, appointing his grandfather Lord Tywin, along with Lord Mace and Prince Oberyn, as judges. Several witnesses are called for the prosecution, all of whom give testimony backing the accusations against Tyrion. Oberyn occasionally questions the witnesses, sometimes insightfully and sometimes (as with Shae) for his own amusement. Oberyn's face suggests interest when Tyrion demands trial by combat.

Some days after his demands for a trial by combat, Oberyn visits Tyrion in the dungeons and relates the story of his disappointment in their first encounter. As children, Oberyn and Elia were told stories of the "Lannister Monster" and his extreme deformities. During a childhood visit to Casterly Rock, Cersei showed Tyrion, with great ceremony, to Oberyn and Elia. Oberyn tells Tyrion that he saw no monster, just a baby. Cersei told Oberyn and his sister that Tyrion killed her mother, and then cruelly assaulted Tyrion until Jaime stopped her. Oberyn tells Tyrion that he seeks justice for the death of his sister, and Tyrion replies that he will find none in King's Landing. Oberyn counters that he is in the perfect place; all those he means to bring to justice for his sister's murder are close at hand. Intending to start his revenge with Gregor Clegane, Oberyn offers his service as Tyrion's champion in the coming trial by combat.

Oberyn enters the trial wearing simple light armor and wielding a long, thin spear. Upon seeing his opponent, Ellaria grows concerned for his chances at besting the mountainous beast, but Oberyn remains confident. Tyrion is additionally worried after witnessing Oberyn drinking before the match, but Oberyn claims that  he does the same before every battle. Before he faces Clegane, he promises that he will emerge victorious to Ellaria. As the battle com menses, the "red viper" manages to impress the crowd with a dazzling display of acrobatic feats before engaging Clegane. Although the latter wears heavier armor and wields a heavier sword, Oberyn is able to swiftly duck and avoid his swings, poking and prodding at him with his spear, 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, who falls over, outmanuevered by the Dornish prince's quick movement and skill with a spear. With Clegane incapacitated, Oberyn once again demands that he admit to Elia's death and reveal who orchestrated it, pointing accusingly at Tywin. However, Oberyn is caught unexpectedly when Clegane trips and pulls him to the ground, grasping him around the neck and slamming his fist squarely into his face. Clegane proceeds to sadistically admit to Elia's murder, before crushing Oberyn's eyeballs with his thumbs and brutally smashing his skull in, killing him instantly as Ellaria looks on in horror, thus officially, through the rules of trial by combat, sealing the fate of Tyrion Lannister.

Quotes
"If the Mountain killed my sister, then your father gave the order."

- Oberyn to Tyrion Lannister

"May I tell you a secret? You're not a golden lion. You're just a pink little man who's far too slow on the draw."

- Oberyn to a Lannister soldier

"The Lannisters aren't the only ones who pay their debts."

- Oberyn to Tyrion Lannister

"In some places, the high-born frown upon those of low birth and in others the rape and murder of women and children is considered distasteful."

- Oberyn to Tywin Lannister

"I have been to Essos and seen the Unsullied firsthand. They are very impressive on the battlefield. Less so in the bedroom."

- Oberyn at a meeting of the Small Council

"It is a big and beautiful world. Most of us live and die in the same corner where we were born and never get to see any of it. I don’t want to be most of us."

- Oberyn to Varys regarding his years in Essos

In the books
In the A Song of Ice and Fire novels, Prince Oberyn is the hot-headed younger brother of Prince Doran Martell. He has eight bastard daughters, called the Sand Snakes, the four youngest of them by his current paramour, Ellaria Sand.

Oberyn has a lined face with thin eyebrows, black "viper" eyes and a sharp nose. His hair is lustrous and black with only a few silver streaks and recedes from his brow into a widow's peak. It is rumored that he has sexual relations with his squires when Ellaria is unavailable. It is not unusual for lords and ladies in Dorne to be openly bisexual, due to the relaxed attitude towards sexuality among the Dornish.

Oberyn is called "the Red Viper" by friend and foe alike due to his penchant for coating his weapons in lethal poisons, so that even a glancing wound will prove fatal.

Although Oberyn is known to have studied to become a Maester in his youth - successfully forging several links of his chain - he is noted to have studied the poisoner's art while travelling the Free Cities, not at the Citadel, as the show specifies.