One thing you may not know about beholders, aside from their extreme xenophobia towards anything and everything, is how creatures so alien as beholders reproduce.
First off, get those dirty images out of your head. They’re way too xenophobic for that.
Turns out, sources actually differ a bit on the how.
Probably because you have to be a certain brand of crazy to even try and find a beholder, much less study one enough in hope of finding out how even more are made.
Volo is one such madman.
In his guide, Volo explains that beholders are such magical beings that they reproduce by dreaming of other beholders, though such dreams are very rare indeed. Reality coalesces/warps/etc, and suddenly you have a new beholder floating near the previous one. Naturally, they both attack each other, but sometimes both will survive.
The question I want to raise is this: What happens to an ecosystem that has species which violate the law of conservation of matter?
Eventually, the beholders die, either to each other, a cave-in, disease, random adventurers, etc. Even the lich beholders eventually crumble into bony dust. Which means more matter, and potentially nutrients, for the ecosystem.
Going back to the post I made on trophic levels, this would make Beholders a primary producer.
Sure, not many things could kill a Beholder, and they’ll likely wind up killing most things themselves. But they still bring energy (and matter) into the ecosystem, even if at a very slow rate.
So what happens with all the extra matter that builds up over the centuries or millennia? Does dirt get pushed above ground to make room for it over time? Is there a secret burial ground for dead beholders?
Eventually, if the population lasted long enough or got far enough out of control, all that extra mass and all those extra complex molecules would start to make some interesting changes to the local environment. Especially on the geological timescale.
And going back to the role as primary producers, you may have other species act as producers for the cave system they live in (through digesting away at rock or harnessing the energy from radioactive minerals), but who’s to say that some brave species of fungus or moss doesn’t start playing parasite to the magical ball of death?
After all, if you can start growing on a beholder and keep it from getting rid of you, then you’ve got it pretty much made. Few other things will be able to touch you.
Perhaps there’s some animals that lie in wait for a beholder to pass by, before striking to inject their young into the beholder’s body, sacrificing their life to do so? The young grow and mature inside of the living creature, feeding on its flesh (saving vital organs for last, much like a tarantula hawk wasp) before eventually burrowing out, killing the beholder in the process.
This idea isn’t new, there’s many parasites that act that way. Fair warning, pretty graphic video I just linked.
Maybe they give the beholder nightmares while they’re inside it. Very specific nightmares, which take advantage of the beholder’s ability to create life from dreams.
Or maybe there’s a fungus that attaches its spores to the bottom of the beholder’s body or to its eyestalks, eventually growing into its brain and controlling its behavior? Again, not unheard of. Such a fungus could make the beholder seek out others of its kind instead of avoid or attack them. Or seek out other species that may help its lifecycle.
I wonder what such a source of highly magical flesh would do to a species of parasite that lived off of it?
Or how that might affect the other animals in the ecosystem, maybe becoming more and more condensed in each trophic level it goes up?
What would the Tertiary consumer look like?