The Sept of Baelor, and a queen crowned in ashes
Cersei Lannister solves the problem of her trial by burning the Great Sept with her accusers, her rivals, and a great many bystanders inside it. Her last living child steps out of a window in response, and she takes the Iron Throne with nothing left to protect and no one left to restrain her.
The fuller tale
Cersei Lannister had spent two years being destroyed by a religious movement she had armed herself, out of a plan to embarrass the Tyrells that had gone the way her plans generally went. She had been imprisoned by it, tried by it, stripped and walked naked the length of her own capital by it, and she was to be tried again.
She did not attend. What she did instead was find the wildfire that had been left under the city since Aerys ÔÇö the caches the Mad King had laid and nobody had ever dug out, under the Sept of Baelor among other places ÔÇö and light it while the entire congregation was inside.
The Great Sept and everything for streets around went up in green fire in a moment: the High Sparrow, the Faith Militant, the queen her son had married, her father and brother, the Grand Maester, hundreds of the faithful and everyone who happened to be walking past. Her son watched it from a window in the Red Keep, took off his crown, and stepped out.
A prophecy closing
As a girl, Cersei had gone to a woods witch in a tent and been told three things: that she would be queen until a younger one cast her down, that she would have three children and see all three buried in gold, and one more that she never repeated to anyone.
By the end of that day she had buried the last of the three, had no younger queen left alive, and was sitting on the Iron Throne with nothing to protect and nobody to restrain her. She had done everything she did to keep the prophecy from coming true, and every step of it was a step toward it.
Elsewhere in the wiki
 See the world as it stood in 302 AC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.