The Season 6 finale of Game of Thrones started out strong, with a catastrophic blaze wiping out several major characters in one fell swoop – but how did the fire start on Game of Thrones? The wildfire left over years ago from The Mad King Aerys' rule was ignited by Cersei as part of her shocking plan to eliminate her enemies.
The Mad King had originally planned to ignite the wildfire to wipe out King's Landing, killing many of his own people in the process, during the rebellion to remove him from the throne. In fact, that deranged decision inadvertently led to the Mad King's own death. Jaime Lannister, then the head of Aerys' Kingsguard, discovered his king's evil plan and killed him in order to prevent the catastrophe.
Of course, few people know the true reason why Jaime killed his king and that he did it to protect them. He became widely known by the derogatory name of "Kingslayer," meant as a pejorative given his betrayal of the man he'd sworn to protect. Few know of the true reason why Jaime killed Aerys – his brother Tyrion, who recently confided the reason for Jaime's betrayal of the Mad King to Aerys' daughter Daenerys, was one of them.
This isn't the first reappearance of Aerys' wildfire. During the Battle of Blackwater back on Game of Thrones Season 2, Tyrion ended up using the stash of uber destructive wildfire in a last ditch effort to defend King's Landing against Stannis Baratheon. Stannis had been sailing towards the capital in order to take the Iron Throne for himself, after the death of his brother (and the last legitimate king) Robert Baratheon.
As it turns out, Tyrion left plenty of wildfire behind – enough for Cersei to wipe out nearly all of her living enemies. Unfortunately, Cersei's shocking plan had unforeseen consequences.
Cersei had her bodyguard, Gregor Clegane, keep her son Tommen from attending the trial, thereby preventing his death. But once Tommen realized what his mother had done, he killed himself by jumping from the tower in which he was sequestered. In addition to killing the High Sparrow, Margaery Tyrell, and Loras Tyrell, Cersei effectively (but indirectly) killed her own son as well.