[world building] two of the energy/elemental planes stop working.

[Particle_Man]

Just spit-balling a save the world campaign where a) the plane of positive energy has been seeded and overwhelmed with dream vestiges from Liberia Mortis, so it stops providing positive energy, and b) the plane of fire has been seeded and overwhelmed with brown mold, so it stops providing fire and heat. I will hand wave questions of infinity aside for the sake of the experiment.

Ok, so the prime material world is in trouble and it will take mighty adventurers to save it with some sort of mcguffins. Meanwhile: healing magic doesn’t work, fire magic doesn’t work, no new sentient life is created (nothing with souls), and no fires can be started.

So what are some other setti consequences for this campaign?

[Particle_Man]

Wait did I accidentally reinvent frostfell ?

[Kelb_Panthera]

Ignoring for a moment that neither of those should logically work that way, one of the more obvious consequences to me would be the sun at least dimming if not being put out altogether.

In many settings the sun is, itself, a hole in the firmament to, or expression of, the elemental plane of fire or positive energy plane if not a confluence of the two providing both warmth and light to the world. This could lead to underdark creatures coming to the surface without having to worry about their sensitivities to light and creatures whose circadian cycles have been disrupted acting strangely. It would also certainly be a blight on the world's plant-life as it can't reproduce anymore than anything else and will start rapidly dying off for lack of sustenance.

Technically speaking, humanoids, or at least humans, should be dying off pretty quickly too since they can no longer cook food and we're actually pretty reliant on that technology for proper digestion. The evolutionary biologists like to phrase it as "we traded guts for brains after discovering how to wield fire." That may or may not apply to any given humanoid race, including humans, depending on how sciency you like your fantasy world to be.

On the note of fantasy biology, how are creatures with the fire subtype, i.e. red dragons, faring in this situation? I'm betting not well. What also of undead who are, in part, manifestations of the negative energy plane according to Libris Mortis?

[icefractal]

Fairly apocalyptic. As Kelb mentions, this should at least dim the sun if not put it out, so at least a reduction in crop growth. Perhaps even more pressing, with no fires the night is an impenetrable void for most people without darkvision. Light spells probably still work (although maybe not?), but those are limited to relatively few people. The climate would probably become colder and colder as well, although that might take longer.

Some enclaves could hold out by using magically produced food and light. Even then, they'd be contending with an outside world that's increasingly undead as things get colder and living things die off. Those who are able may flee to non-material planes like the Astral where the effects are less significant.

Long term, the lack of positive energy may have the biggest impact across planes. Does this block fast healing too? If so, undead probably take over. If not, they'd still grow massively in strength and number.

[nedz]

In many settings the sun is, itself, a hole in the firmament to, or expression of, the elemental plane of fire or positive energy plane if not a confluence of the two providing both warmth and light to the world. This could lead to underdark creatures coming to the surface without having to worry about their sensitivities to light and creatures whose circadian cycles have been disrupted acting strangely. It would also certainly be a blight on the world's plant-life as it can't reproduce anymore than anything else and will start rapidly dying off for lack of sustenance.

So Darksun rather than Frostfell then ?

On the infinities question: you could just have the infestations relate to the border regions between (this) Prime Material and the Elemental planes.

You could also stagger the effects with, say, Fire first and then the Positive. This would create some tension as the player's consider "Which plane will be next ?" - even if that never happens.