What Is the Green Fire in 'Game of Thrones' Season 8, Episode 5?

In Game of Thrones Season 5, Episode 8, "The Bells," Daenerys Targaryen (Emilia Clarke) fulfills a threat that's loomed over Westeros since she first came ashore at Dragonstone with her three dragons, burning down King's Landing and slaughtering not just Lannister soldiers, but thousands of innocent people as well. It was a sharp turn for Daenerys' character, but we'll have to wait for the final episode of Game of Thrones to see the consequences. But amid Daenerys' dragon rampage, eagle-eyed viewers noticed a different type of fire than that emitted by Drogon. So what are the green flames that burst from the ground all over King's Landing during the assault?

Wildfire, which hasn't appeared on Game of Thrones since Season 6, when Queen Cersei (Lena Headey) used the volatile substance to blow up Baelor's Sept, killing Queen Margaery Tyrell (Natalie Dormer) and securing the Iron Throne for herself.

What is the Green Fire in Game of Thrones Season 8, Episode 5

Wildfire is a flammable fluid, created by the pyromancers of the Alchemist's Guild, who Tyrion (Peter Dinklage) pays a visit to before the Battle of the Blackwater, all the way back in Game of Thrones Season 2.

"I remember reading an old sailor's proverb," Tyrion tells the pyromancer Hallyne (Roy Dotrice), "Piss on wildfire and your cock burns off."

"I've not conducted this experiment. It could well be true," Hallyne, apparently a quite literal man, replies. "It could well be true. The substance burns so hot it melts wood, stone, even steel! And, of course, flesh. The substance burns so hot it melts flesh like tallow. After the dragons died, wildfire was the key to Targaryen power."

It was wildfire that Daenerys' father, the Mad King Aerys II Targaryen, used to burn his enemies. He had even plotted to burn King's Landing to the ground with hidden caches of it — a plan foiled by Jaime Lannister, the Kingslayer.

While no clear explanation is given for exactly why wildfire explodes all over King's Landing during Game of Thrones Season 8, Episode 5, it's reasonable to assume these are Cersei's underground stockpiles, exploded by Drogon's dragonfire. The explosions are more than cool, green fire visuals; they also serve as a reminder of how Daenerys' destruction of King's Landing mirrors the mental breakdown of her father before her.

We'll see if Daenerys takes the hint, when Game of Thrones returns for its final episode on May 19.