Before scientific experimentation disproved it, most people believed in the spontaneous generation of life (also known as equivocal generation). For example, it was believed the maggots spontaneously generated out of rotting meat. That places of darkness spontaneously generated all manner of crawling things (as could be seen upon lifting up a rock). Worms arose from dirt and even frogs were once thought to come from the mud.
This was a universe literally teeming with life energy.
For the purposes of fantasy, I find it particularly fascinating how many of these theories revolved around places of filth, putrescence, and decay giving rise to vermin: Maggots from rotting flesh, rats from excrement, and so forth.
So when we look around our fantasy milieus and see monstrous forms of vermin — dire rats, giant spiders, and the like — could we use such theories of creation as their basis? I think we can. In the vile places of the earth — the places were the very form of the world itself has begun to decay — such creatures are given form out of the darkness. And thus our dungeons teem with life.
Another interesting facet of the theory of the spontaneous generation of life is that, like many pre-scientific beliefs, it was not extinguished by scientific thought — rather, it retreated before it. Once it was proven that maggots are the larvae of flies, for example, the theory simply moved to become the spontaneous generation of bacteria out of the “life force” of the air. It wasn’t until Louis Pasteur rooted out the last of its hiding places that the theory finally died.
I bring this up to suggest that we can just as easily turn the dial in the opposite direction. We can even add a metaphorical (or perhaps metaphysical) component to the theory.
For example, John Wick’s Orkworld postulates that orcs are actually photosynthetic. But we could just as easily say that the orcs spontaneously arise wherever civilization is not: When empires fall, tribes of orcs appear in the ruins.
And we could elaborate upon further upon the themes of this theory: Mice were known to physically reproduce, but they were also thought to spontaneously generate out of moldy grain. Ergo, our orcs can both spontaneously generate out of the lack of civilization, but also reproduce.
Further, we could build upon this by considering the ideas of Lamarckian evolution — that life, once spontaneously generated, strives to become more and more complex. Thus we could postulate that all humanoid life has its origins in the primordial generation of orcish life from the absence of civilization — orcs, goblins, hobgoblins, and beastmen of all sorts appear spontaneously, but then some among them will breed and improve.
This doesn’t even necessarily need to be true, but it would certainly be an interesting religious belief. For example, your dark elves might believe that orcs, humans, elves, and dark elves all exist along a continuum of improvement. (It might be even more shocking if you made this a secret belief among the elves: They look at humans the same way that humans look at orcs… they’re just too polite to say it.)
You could root all of this cosmology in the apeironic weave of creation. Where that weave becomes weakened, spontaneous generation occurs. When weakened by decay, filth and vermin appear. When weakened by the loss of civilization, orcs appear. When weakened by vile death, undead appear.
Such a cosmology could have interesting implications for how summoning rituals or the creation of undead work. For example, Jean Baptista von Helmont had a receipe for mice: “Place a dirty shirt or some rags in an open pot or barrel containing a few grains of wheat or some wheat bran, and in 21 days, mice will appear.” Magical spells like create undead could directly manipulate the apeironic weave in order to cause undead to appear; but you could also open things up by allowing non-magical manipulation of that weave (through ritualistic murder, for example).
You could also flip the whole conceit of “weakening the weave” on its head. Perhaps clerics and living saints appear in those places where the weave has been strengthened. Perhaps the gods themselves are nothing more than those places where the weave has been unnaturally strengthened through the power of belief or sacrifice or chance.