You might be looking for Keeper of the Lost Cities Book Three: Everblaze.
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This article contains plot details from the latest books.
Everblaze is an unstoppable flame that burns a neon yellow color. It is said to smell like burning sugar and be white-hot. It also can burn Pyrokinetics who are otherwise fireproof due to their ability. Even Pyrokinetics cannot truly control it, because it's so unpredictable. 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 that Sophie had once accidentally bottled. In Book 1, 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. This results in a tribunal for illegally leaping to the Forbidden Cities. In book three, Fintan ignites the Everblaze when Sophie is in the process of healing his mind. Before enough Frissyn was created, it killed Kenric and destroyed much of Eternalia.