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.
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". He is nicknamed "the Red Viper" for his knowledge of poisons and for his unusual, deadly style of combat. 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; she is his wife in all but name. He has eight acknowledged bastard daughters, the last four born to Ellaria Sand, all of whom he has loving relationships with. He named the fifth one, Elia Sand, after his late sister.
When he was a teenager, Oberyn was accompianed by his little sister Elia on a visit to Casterly Rock. While there, Oberyn and Elia were eager to see the "monster" that had just been born. Cersei finally showed them Tyrion, but the Martell siblings were somewhat disappointed to see that he was just a baby, and somewhat disturbed by Cersei's already strong hatred of him.
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.
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 Olyvar 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.
n the days following Joffrey's death, Oberyn and Ellaria visit the brothel again. Oberyn grows angry with Olyvar 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 gender and having as much sex as possible before old age) with Olyvar. 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 successfully resisted 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 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 light spear with a heavy blade. 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 commenses, the "red viper" manages to impress the crowd with a dazzling display of spear twirling before engaging Clegane, whom he taunts. Clegane ignores his words and launches a furious attack. Although the latter wears heavier armor and wields a massive broadsword, 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, vengefully chanting in an ever-louder voice: "You raped her! You murdered her! You killed her children!". Clegane knocks Oberyn off his feet twice and manages to cut his spear in half, but fails to wound him; Oberyn simply evades his swings again and walks over to his squire, who tosses him a replacement spear. Meanwhile, he wears Clegane down, knocking off his helmet and inflicting minor wounds through gaps in the huge knights' armor.
Oberyn eventually stabs Clegane in the stomach and severs his hamstring. The knight falls to his knees, outmaneuvered by the Dornish prince's quick movement and skill with a spear. The Red Viper then lunges and stabs him through his breastplate, causing Clegane to cough blood and fall on his back. 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 -stalking aggressively around his unmoving opponent- is caught unexpectedly when Clegane trips him and pulls him to the ground, grasping him around the neck and slamming his fist squarely into his face, knocking most of his teeth out in the process. Clegane proceeds to gouge out Oberyn's eyeballs with his thumbs, and while Oberyn screams in agony, Clegane sadistically confesses to raping Elia, killing her children and then killing Elia herself. He brutally crushes Oberyn's skull, killing him instantly, as Ellaria screams in horror. Thus, through the rules of trial by combat, the fate of Tyrion Lannister is sealed.
While the Mountain is being treated for his injuries, Pycelle and Qyburn discover that before the duel Oberyn had his spearblade coated with Manticore venom, which has caused Clegane's already severe wounds to putrefy horribly, leaving him in a catatonic, decaying and agonized state.
A younger prince of Dorne, Oberyn Martell was also a fierce fighter who earned the nickname the "Red Viper". He spent time in the Citadel but quit his studies before earning a full set of links. Prince Oberyn acted as Tyrion Lannister's champion during Tyrion's trial by combat for the murder of King Joffrey. He was killed by Gregor Clegane.
Oberyn Martell is a forceful, lusty man with a quick wit and barbed tongue. He is renowned for his duels, his temper, his keen intellect, and his carnality. Oberyn traveled the world in his youth, and even fought in a mercenary company in Essos - his skill with a spear is enough to face off evenly against the feared Gregor Clegane in a duel. He had a very close relationship with his sister Elia and they were inseparable as children. Oberyn is also a very intelligent man: he studied at the Citadel for several years before deciding he was simply bored with a future as a maester and left. During that time he studied poisons extensively, which he frequently coats his weapons with.
Above all Oberyn is a deeply passionate man, with a hot temper that often leads him into duels and grudges, but in equal measure this also makes him deeply loyal to those he cares about, such as his siblings, his paramour Ellaria, and his daughters.
Oberyn's overall attitude is that life is short, and one must enjoy all of life's pleasures while one can - thus his sexual adventures have become infamous, and popular rumor has it that he must have had sex with half of the people he has ever met on both sides of the Narrow Sea. Oberyn is a deeply honorable man, but his philosophy in life is that he doesn't care about the negative opinions of others - he would rather live life on his own terms, openly challenging his enemies and risk destruction rather than live by someone else's rules.