You might be looking for Keeper of the Lost Cities Book Three: Everblaze.
| Spoiler warning!|
This article contains plot details from the latest books.
Everblaze is an unstoppable flame that burns a neon yellow color. Even Pyrokinetics cannot truly control it. It is very dangerous, and the only thing that can stop it is a formula called Frissyn, which contains Quintessence, the light of an unmapped star. Sophie bottles the Everblaze with Lumenite and gold, in a short, round bottle with a fluted neck and a wide opening, in comparison to other fire-catching bottles, which are long and thin. In book three, Fintan ignites the Everblaze when Sophie heals his mind. Before enough Fryssyn is created, it kills Kenric and destroys much of Eternalia.