What is the Valonqar prophecy?
The Valonqar part isn't actually included in that flashback. Instead, it's only in Martin's book A Feast of Crows. Young Cersei asks if she'll have children, and Maggy the Frog responds with the following menacing prediction:
"Oh, aye. Six-and-ten for him, and three for you. Gold shall be their crowns and gold their shrouds … And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you."
Now, what exactly does "valonqar" mean in this context? It's the High Valyrian word for "little brother," which is why many have speculated that Tyrion would be the one to ultimately kill his sister. (This would also partially explain the suspicion and contempt Cersei has toward her Tyrion throughout the series.) Another interpretation was that Jaime was in fact the valonqar, and that he would be the one to kill her. As Jaime became increasingly righteous and good, that felt like the likeliest path heading into "The Bells."
Obviously, Cersei wasn't strangled to death by Jaime last night. However, that doesn't necessarily mean that the valonqar theory was total bunk or a piece of misdirection. It only suggests something careful viewers have always known: prophecies are merely road-maps and blueprints for potential future outcomes. More specifically, they often require interpretation and thrive on ambiguity. While it's easy to see a deviation like Jaime not killing Cersei as a "cheat," I'd instead argue that the "payoff" speaks to the slippery purpose of a prophecy in a story like this. The pleasure comes from the swerve, not from seeing another box checked off.
It's possible to interpret the valonqar theory as mostly correct. When Cersei is attempting to flee King's Landing at the end of "The Bells," she is crying quite heavily. She finally knows the end is near. Her power is literally crumbling around her, giant boulders falling from the sky and dust gathering on the ground. After the brutal death of Qyburn and the fiery demise of The Mountain, she has lost her two remaining advisors. Jaime, her twin (albeit slightly younger) brother and the father of her children, does wrap his hands around her throat as she dies, but it was in a moment of love, a final act of devotion before the falling rubble ended her life. It's far more poignant than a more blood-thirsty, ruthless final image would have been.
In an interview with Entertainment Weekly, actress Lena Heady described her own complicated reaction to learning her character's fate. At first, she says she had "mixed" feelings about it, but that she came around to the idea after talking it over with her costar Nikolaj Coster-Waldau. "I think the biggest surprise is he came back for her," she said. "Cersei realizes just how she loves him and just how much he loves her. It’s the most authentic connection she’s ever had. Ultimately they belong together.”
A brilliant schemer and fearsome leader, Cersei Lannister was one of the most formidable and complicated characters in the series. Like most members of her family, she was motivated by a quest for total power but also by a desire to protect the ones she loved. For the most part, the quickened pace of Season 8 has meant that we haven't spent much time with Queen Cersei: Her most significant plotline this season was her questionable romance with the skeevy pirate dude Euron Greyjoy, who ended up stabbing Jaime on the beach and dying on the rocks. Within the confines of King's Landing, Cersei simply didn't have any formidable rivals left, giving her scenes an airless quality and a lack of tension. But even walled in by the storyline and the prophecy, the character still got a surprising final moment to end on.