At one time it was thought that the substance known as Anagra Estratius, or Devouring Fire, was an alchemical substance, tainted by the infernal components that went into its making. It is now known not to be the truth. It has been found to be indeed an infernal creature. Somewhat base perhaps, but they have their uses.
The process for creating this substance is macabre and somewhat unsophisticated in its nature . We need not go into too much detail, save that it involves effluent from a Brass Bull with the more pure a 'subject' the better. The process for creating the Anagra not in truth creation, but a summons. Little more is needed beyond the rendered oil and the desire to bring forth the Anagra - any words or rituals purely serve as a focus and their details are largely irrelevant. The Anagra want to come.
The Anagra can choose to burn only the small volume it touches, or it can - more typically, ignite massive, uncontrolled fires. Regardless of the size of the fire it causes, no Anagra has been seen to grow larger then would be needed to engulf a human. Anagra can move, but only from flammable to flammable materials. Vegetation, no matter how green, is considered to be flammable, as is flesh.
This is not in fact true - Anagra have even consumed a Dragon, though that was a willing sacrifice.
The fire, which so readily spreads, is in fact an infernal spirit. It appears as a unusually active yellow-red flame that can change colors from time to time, in deference to the Anagra's temperament. Unholy glee sees the flame burn intense scarlet. Fear - rare but possible - a near white straw-colored flame. When it is foul and brooding, or when it decides to enshroud a mortal, it is a sickening green.
I mentioned the enshrouding of a mortal. The Anagra is capable of burning a mortal spiritually while leaving their body mostly intact. The process causes the soul of the mortal to either be devoured and merged with the Anagra, or to flee to whatever final fate awaits it. In its place, part or all of the Angara remains in the body, which is now available for its use. The possession typically gives the body an unpleasant, greenish cast, but also grant demonic strength and durability to the form. Those only partially possessed are held in thrall by the Anagra, with rudiments of the being's old personality and knowledge remaining in what are little more then zombie forms. These can survive in this form for several days before crumbling into ash. Fully possessed individuals instead have a halo of flame that leaves the being's flesh and possessions uncharred, and give the Anagra hands and voice to interact with our world. A body can only contain the full Anagra for a few hours before a sudden and complete incineration. Until such time, enshrouded beings are very resistant to mundane fire.
As one would expect, water and smothering are agents effective at controlling an Anagra, but are not sufficient to destroy it or banish it from our realm. The fires it ignites can be put out normally, though perhaps with difficulty, considering how thorough it usually immolates its surroundings. Water allied with divine enchantment is its main weakness, though other magic can inflict pain upon it.
Anagra can be contained as would most fires - with metal or stone, ideally fortified by magic. However, the greatest care must be made to keep unprotected flammable materials well away from it, for should it ignite it, it will leave whatever bounds it has in a flash.
The Anagra are in fact manifestations of Gluttony, and is a child of a greater demon yet unnamed and undocumented. It exists to consume without creation. It leaves not even ash from what it burns. An Anagra holds no love for its summoner, who must bind it to his service by magic or by cunning containment. There are tales of some who have managed to hold it by promise, or cow it with fear, but those have not been substantiated. Occasionally Anagra display greater complexity to their motives than simple lust to consume, but these are rare indeed.
This foul device has an Anagra bound to with strong sorcerous bonds. Svedrian uses it specifically to enshroud his victims, using them as short-lived troops. The Anagra within the torch is capable of maintaining up to 20 concurrent victims, but may enshroud a victim no more frequently than 1 per minute. The torch overrides the Anagra's control of the thralls and also prevents it from fully enshrouding them as well.
The Anagra is more abstract than typical demons, and many who have been victimized by them had no idea they were dealing with the infernal.
Some structure of great importance to the PCs is afire and they plunge in, attempting to combat it, or rescue someone trapped within. They quickly find that it is no ordinary fire they are battling, and they may encounter enshrouded victims within the flames.
Children of the Flame
The PCs find an isolated tribe during their travels, who happen to worship one of these creatures bound to a massive brazier. The temple has a decidedly Indiana Jones feel to it. Sacrifices are thrown onto the brazier and are usually simply burned to nothingness. Occasionally they are Enshrouded and these are seen as 'test's for the tribes warriors who usually are able to destroy them.
The Fire's Secret
The PCs need to deal with one of these things, due to special knowledge that it has. This one is ancient and has enemies, and so requests one of these be captured, alive, and brought to him. Of course, this enemy will be by no means weak, nor would his or her disappearance be unnoticed. And of course, the entire thing is a trap.
For influences, one can seek Howl's Moving Castle and the classic Heavy Metal..