With her brother stamping out the last vestiges of rebellion in the Riverlands, Cersei Lannister's only remaining ally in King's Landing is Qyburn, the defrocked maester and current Master of Whisperers. Since coming to King's Landing with Jamie back in Season 3, Qyburn has been invaluable to Cersei keeping hold of the city. He replaced Varys after the eunuch fled King's Landing with Tyrion after her brother murdered Twyin and saved Gregor Clegane from certain death...although it's unclear whether the hulking Clegane is truly alive or an undead monster.
Two episodes ago, Qyburn gave Cersei a much needed ray of hope after she learned her son had banned the practice of trial by combat, meaning Cersei would face a trial in front of seven judges, all of whom would probably be loyal to Cersei's political rival, the High Sparrow. As Cersei reeled from the announcement, Qyburn approached her and said that his network of spies had discovered that an unnamed rumor she had investigated was much more than a rumor.
So what secret did Qyburn discover? The popular theory is that Qyburn discovered the large cache of wildfire gathered by the Mad King Aerys. Wildfire was first introduced in Season 2, when Tyrion used the flammable liquid during the Battle of Blackwater Bay. Tyrion lit the entire bay on fire, engulfing Stannis Baratheon's fleet in flames and killing most of Stannis's army.
While Tyrion had supposedly used the city's entire supply of wildfire, Jamie Lannister offhandedly mentioned to Brienne that Aerys had stored wildfire throughout King's Landing and he planned to burn the city to the ground when the Lannister army invaded the city during Robert's Rebellion. Jamie murdered Aerys to prevent the king from killing thousands of innocents, earning the title of "Kingslayer". Among the many places Aerys had stored his wildfire was under the Great Sept, the spot where Cersei will have her trial in "The Winds of Winter".
While it might seem odd that Game of Thrones is bringing back wildfire after not mentioning it for several years, producers may have snuck in a clue to its importance in "The Winds of Winter". One of Bran's most recent visions included a glimpse of a shadowy figure in chains holding a large container of wildfire. While it's possible this is a vision of the past, the chains indicate it could be Cersei holding the wildfire in her hands.
So what would Qyburn and Cersei do with the wildfire? Well, the High Sparrow, his Faith Militant and the Tyrells will almost certainly all be in attendance at her trial. Could Cersei take a page out of Daenerys's book and trap her enemies alive in a burning building? That seems pretty savage, even for Cersei. But many of Game of Thrones most shocking moments have involved Cersei, so don't be surprised if she uses whatever Qyburn has dug up to her advantage.