Conflagration
In your world the components of FIRE (conflagration) are natural forces governed by physical science. Flame is the first stage; Smoke, the unburnt particles is the second stage; and the third stage developed by the intensity of the heat in Smoke -is the most powerful and the most feared of all - Flashover.
In the Fire Engineers world there is another factor unknown in your world. People witnessing fires have felt something. Something more than the physical nature of the fire phenomena. They have felt a presence. Some even claim to have seen figures in the fire; and some say the people who have seen these things are mad.
But some say they are manifestations of the entities who roamed the Earth long ago. For the time has come for the Ancient Ones to return and claim the Earth once again.
The Fire Engineer does not believe in superstition; but he has seen these phenomena, and felt them. And what he felt was the heart of darkness.