Ellaria Sand
Oberyn's paramour, a bastard of House Uller and mother to several of his Sand Snakes — in the show, as ready for vengeance as any of them.
A soul of House Martell, born at Hellholt.
The fuller tale
Season Four
Oberyn's paramour and, by her own account, happier being his lover than being anyone's wife — she comes to King's Landing with him for Joffrey's wedding and stays close through the brothel scenes and the small council needling, plainly enjoying how much discomfort the two of them cause the Lannisters just by being unimpressed. She backs his obsession with Elia without ever quite talking him out of the parts of it that will get him killed, and when he volunteers to fight Gregor Clegane she is there to watch, certain he will win right up until the moment he does not. His death leaves her in the crowd, holding nothing, while the vengeance the whole trip was supposed to deliver goes up in a spray of blood that is entirely his own.
Season Five
Grief has curdled into something harder by the time Dorne reopens the story: Ellaria wants the Lannisters to bleed for Oberyn and has no patience left for Doran's decades of careful peace-keeping. She falls in with Oberyn's daughters, the Sand Snakes, and the group of them work against their own prince as much as against King's Landing, furious that Myrcella Baratheon gets to live safely in Dorne as a hostage-turned-guest while Oberyn rots in the ground. It is Ellaria who finally gets close enough to do something about it, kissing Myrcella goodbye at the docks with poison on her lips — a slow-acting revenge dressed up as tenderness, set loose just as the girl's ship pulls away.
Season Six
Ellaria stops waiting on Doran's patience and ends it herself, working with the Sand Snakes to kill both the prince and his son Trystane within the same night — Doran in his own chambers, Trystane on the ship still carrying him home. She takes Dorne for her own with the Sand Snakes at her side, and by the end of the season has traded her old grievance against the crown for a seat in a much bigger conspiracy, offering Dorne's spears and ships to the alliance gathering behind Daenerys Targaryen.
Season Seven
Dorne throws its fleet in with the grand alliance sailing for Westeros, and Ellaria sails with it, her daughter Tyene at her side and old scores apparently settled. Euron Greyjoy has other plans: his fleet catches the Dornish ships at sea, and Ellaria and Tyene are taken alive and delivered to Cersei as payback for Myrcella dressed up as a gift. Cersei returns the favor from two seasons back with interest, poisoning Tyene with the same slow venom Ellaria used on Myrcella and chaining Ellaria in a cell to watch her daughter die by inches, close enough to talk to and powerless to help. It is the same cruelty Ellaria dealt out at the docks, aimed back at her exactly once and left to work.
How it ends for them
The chronicle records no death for this soul — they are living when the tale leaves them, or their ending is a page the maesters have yet to turn.
Their chapters, book by book
Oberyn's paramour, a bastard of House Uller and mother to four of his Sand Snakes — who, in the books, is the one voice begging Dorne AGAINST the war of vengeance: she has seen where it ends.
Where the page and the screen part
No character was more completely inverted by adaptation. Book Ellaria is the voice against the war of vengeance — her 'where does it end' speech to Doran is the books' moral summary of the entire revenge theme — while show Ellaria murders a princess, a prince and Dorne's line of succession to start that very war, a rewrite so total that the two versions share only a name, a lover and a birthplace. The show also compresses the family tree: its Ellaria is mother-figure to all three Sand Snakes, where the books give her only the four youngest of eight, none of the three the show kept. Her show fate — chained in the Red Keep's black cells, condemned to watch her poisoned daughter's fate at close, permanent range — is Cersei's mirror-justice for Myrcella and has, like everything else in the arc, no book counterpart at all.
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.