As is well known, there are three elements to combustion: temperature, fuel, and oxygen.
Although the wizarding world possesses magic, it still obeys this rule. While Fiendfyre is extremely powerful—so much so that ordinary means cannot extinguish it at all—it still adheres to these three elements.
Therefore, as the Incendio blazed furiously, the oxygen inside the protective barrier began to drop sharply.
Chapter 391