It's hard to keep track of all the times that Cersei Lannister on Game of Thrones has threatened to "burn cities to the ash," in order to protect her children. According to a few clues sprinkled throughout Season 6, she may just make good on that promise before the season finale. In Episode 8, disgraced maester Qyburn tells Cersei that the rumors she asked him to investigate are true and "more." The internet speculates that the rumors Cersei is wondering about refer to wildfire, which she may or may not use to blow up all of King's Landing.
Early in the season, Bran had a cryptic vision, some of which featured a raging green fire tearing through what appeared to be King's Landing. This could have been a vision of the past from Mad King Aerys' reign, or it could be a vision of the future, a vision of what Cersei has planned. Under the High Sparrow and the Faith Militant's influence, King Tommen has abolished trial by combat. This means that Cersei can no longer count on The Mountain as her champion to guarantee her freedom, and must now stand trial. The decree is bad for Cersei, but probably good for society as a whole. But as we see from her chat with Qyburn, it looks like Cersei has a backup plan.
After learning that her son has left her fate in the hands of the Faith Militant, the charming group that organized her Shame Walk, Cersei is pretty much out of options and out of allies. When backed into a corner, Cersei does not have the most tactical mind, and has been known to act impulsively. As she told the members of the Faith Militant who tried to get her to obey them: "I choose violence." It's likely that Cersei will choose violence again in order to protect herself and her last remaining child, King Tommen.
Wildfire is an explosive substance that King Aerys reportedly ordered his alchemists to make tons of before he was executed by Jaime. The plan was to, quite literally, burn the city to the ground. Rumor has it that the king ordered dozens of caches of wildfire to be hidden throughout the city. If the theories are correct, those caches still exist, and Cersei is planning to use them to blow up the Faith Militant. But the thing about wildfire is that it's, well, wild, and almost impossible to control. If this is indeed Cersei's plan, the collateral damage could be huge, and might even end up killing her son.