This is an extension of the earlier theory that Cersei will use Wildfire in King’s Landing to blow up the Sept of Baelor. This extended theory is credited to reddit user JoeMagician.
“In [HBO’s “Game of Thrones” Season 6 Episode 8], there was an odd scene after Tommen declared that there will be no more trials by combat. Cersei has a whispered conversation with Qyburn and asks about a rumor she had his spies investigate. Qyburn confirms that there is substance to those rumors. So what is the rumor? Wildfire under the Sept of Baelor.
When King Aerys was stabbed by Jaime, his last command to the pyromancers was to “Burn them all”. He had, in secret, planted enormous caches of the extremely explosive substance known as wildfire under major buildings all through-out King’s Landing.
From ASOS Jaime V:
He saw traitors everywhere, and Varys was always there to point out any he might have missed. So His Grace commanded his alchemists to place caches of wildfire all over King’s Landing. Beneath Baelor’s Sept and the hovels of Flea Bottom, under stables and storehouses, at all seven gates, even in the cellars of the Red Keep itself.
Jaime never told anybody that the Mad King intended to burn King’s Landing to the ground, keeping his secret until his bathtime fun with Brienne at Harrenhall. The scene and the information runs the same in the books as the show, Jaime never told anyone there was wildfire everywhere and as far as I can tell, it’s never been discovered or removed since the Mad King’s death. The substance is all still there, undiscovered and lethal.
Why would Cersei be interested though? Many remember Tyrion for the wildfire trap he sprung on Stannis at the Blackwater, however it was Cersei who first had the plan. Tyrion got the information from his cousin Lancel that Cersei had ordered thousands upon thousands of pots for the city’s defense. Wildfire is more on Cersei Lannister’s mind than anyone else.
From the books as well, Cersei had the Tower of the Hand exploded by wildfire after Tywin’s death in a scene that troubles Jaime for similarities to the Mad King.
A new stepfather, most like. Jaime knew the look in his sister’s eyes. He had seen it before, most recently on the night of Tommen’s wedding, when she burned the Tower of the Hand. The green light of the wildfire had bathed the face of the watchers, so they looked like nothing so much as rotting corpses, a pack of gleeful ghouls, but some of the corpses were prettier than others. Even in the baleful glow, Cersei had been beautiful to look upon. She’d stood with one hand on her breast, her lips parted, her green eyes shining. She is crying, Jaime had realized, but whether it was from grief or ecstasy he could not have said.
-AFFC, Jaime II
With Gregor Clegane unable to help her in a trial by combat, and out of options, Cersei may take the most drastic step imaginable and blow up the Sept of Baelor and possibly most of King’s Landing rather than face the Faith’s trial. This would also explain why in Bran’s visions, he saw the Mad King commanding the destruction of King’s Landing and the wildfire jars hidden underground. The city is still primed to blow.”
The Mad Queen Cersei: Jaime said Cersei would burn cities to ash
When speaking with Edmure Tully, Jaime professed his admiration for Catelyn Stark and Cersei Lannister. He said of Cersei that she would “burn cities to ash” for her children. That very might might have been foreshadowing.
Bran saw visions of Wildfire exploding in King’s Landing
These visions were covered more extensively in this iteration of the theory. Bran saw visions of the past (the Mad King Aerys screaming “burn them all!” and Jaime killing him before he could set off the Wildfire) and potentially of the future (the Mad Queen Cersei actually setting off the Wildfire?).
Dany will find King’s Landing amidst rubble from the explosion
So what would it mean if Cersei were to blow up King’s Landing with Wildfire? In addition to another Bran vision in which it looks like King’s Landing is covered in smoke, we have a vision from Season 2 when Dany was in the House of the Undying.
In that vision, Dany approaches the Iron Thrones amidst ash, snow and rubble. Could it be that that’s literally how she will see King’s Landing when she finally gets there?
Jaime will kill Cersei
If this theory is true, then the implications are far-reaching. Jaime killed the Mad King Aerys to protect the lives of all of the innocent people. This shows that he does have a semblance of a moral compass. Will he be forced to save the lives of innocent people again — but this time, instead of killing the King, he’d have to kill his sister and lover: the Mad Queen Cersei? If so, that would make the theory that Jaime is Cersei’s Valonqar true.