The Phlogiston

The phlogiston is where stars are born. Huge nebulae of it, hundreds of lightyears across, drift through the astral sea. Within these clouds, the fundamental magic of the universe coalesces into stars. This process takes millions of years, after which the phlogiston in the immediate vicinity of the new star will be burned away by the star's birth.
A phlogiston nebula is formed when the death of an especially large star releases huge quantities of magic into the astral sea around it. The other forms of magic the star has absorbed during its lifetime returns to its origin - the elemental planes for elemental magic, the Core for draconic magic, the outer planes for divine or fiendish magic, etc. Only the fundamental magic of the material plane remains. This state of raw magical energy does not last for long, however. It quickly begins to draw in other nearby sources of magic, forming a volatile and highly flammable gas.
If exposed to any major energy source, such as fire or electricity, phlogiston will very quickly explode. The size of the explosion will depend on the size of the energy source, ranging for a small burst of heat for a candle, to a devastating explosion that can easily wreck a ship from the firing of a cannon. For this reason, phlogiston is usually avoided by spelljammers, as they are largely defenceless in it. This is exactly why many pirate ports and black market trading halls are situated here, away from the prying eyes and vengeful guns of the galaxy's law enforcement.

The Phlogiston in Play:

Any effect that deals fire, lightning, or radiant damage forces every creature within a number of feet equal to the damage dealt to make a DC 15 Dexterity saving throw, taking the damage dealt by this effect on a failure, or half as much on a success. Any effect that deals force, necrotic, or thunder damage forces every creature within a number of feet equal to half the damage dealt to make a DC 12 Dexterity saving throw, taking half damage dealt by this effect on a failure, or one quarter of the damage on a success.