Interesting.
Well, here's what I believe would happen in a diminished-light world, along with a bunch of interesting postulation...
Well, it's not the fact that you don't have 'no sun for a few centuries'. From what you say, from your initial comment is that it is a broken... eaten sun. So, I would imagine that it is exploded? Well, the first thing that comes to mind is that... yes... it would be darker. But with an exploded sun, the sunsets and sunrises would be spectacular. Aurora Borealis on LSD. Or perhaps as violent as Venus' great lightning storms.
Now did you want diminished light? Or no light at all? Hmm, reminds me a bit of the movie Pitch Black. If there is no light at all, a lot of people, who can't see in the dark are going to be in a lot of trouble.
This decreased light amount would cause all sorts of flora and fauna to die off in a near-extinction, in the beginning... Logically it would cause the planet to drop in temperature. As to how much is really up to you. Some people think that if you move our planet away from the sun just a little bit, it would cause the planet to drop in temperature enough that we'd be knocked back into an ice-age again. So, if you want to have an icy world... or at least large icy places of permanent-winter. Well, it's up to you.
Plants that need a lot of light would die off, like large trees, and especially broad-leaf, fruit-bearing trees. There would be lots of land-slides or top soil first blowing away creating vast dust bowls as vegetation died off. This would seriously affect animals and monstrous populations. So, yeah, lots of plants and animals and populations of races/monsters would die off. It might not be overnight, but famine and disease would set in.
No big trees, no wood. No ships or multi-story buildings. Just have to look at some cultures that have no wood in our history. Pick one, there are a few.
What changes does it bring to the weather? A cooling would probably slow-down jet streams. Perhaps the world is a place of stagnating weather patterns. Or something unforeseen caused the world to have more violent weather patterns. Or by now, everything has settled itself out, but occasionally you get these extreme weather conditions. I'm imagining icy-wasteland deserts. Water water everywhere. But it's dirty salty ice. Very cold though, the sky is filled with dancing auroras and purple-magenta lightning storms.
Why would there be a band of scorching desert around the equator? Perhaps I'm missing something, but I would think that with a temperature drop, the equator region would perhaps now become a 'cool desert'... yeah... not many plants, okay... I get it. With all the water locked up in glaciers and ice-sheets. -- A lot of creatures would learn to adapt to live on the edge of these two regions. Giving them a source of water... but also treacherous.
So, of course the ecology would change dramatically. Perhaps, with lots of trees dying off, this allows ferns and dark-loving plants to flourish. Also, I think that after an eon, the predators would have adapted (well, everything would), but what if the predators really became prevalent. Making various wilderness regions very dangerous.
Or... you could go the opposite way and little or no wilderness at all. Most of it is in wintery, deadlands. Personally, I'd try to come up with some interesting, new, wintery habitats, and the 'sudden winter'... or perhaps even a 'big chunk of sun' smashed into the world/planet. This fractured the planet, causing all sorts of tectonic plates to jar and move (not to mention the direct damage it did to the planet). Causing great chains of volcanoes to spring up and creating a thicker more protective atmosphere, combined with their ground generated heat in various locations. Pockets of 'normal' vegetation have sprung up. Or the volcanoes are holding back the icy wastes.
So, perhaps you might end up with the idea that most populations live in regions that have volcanoes bordering icy wastes. Or live along the borders of the deserts and the ice-sheets. Are your intelligent creatures nomadic? If they are nomadic, what makes them so? Do they follow food-stocks of animals? Or do they pick one geographical region that will help them sustain life. Allowing them farming. -- You seem to be thinking on similar lines as well as I am. I too thought that it would drive populations underground if the surface is too harsh an environment to live in. I always toyed with the idea of underground societies living on fungi. Food, lightning, oxygen production. (I'm mostly brainstorming without reading your post, so your ideas don't cloud my ideas... imagining what I would do first. So, please forgive me if I repeat a lot of what you thought of already.)
Perhaps with this apocalyptic event, this drove most 'underdark?' races onto the surface. Do you want to use darkelves? But most definitely, it would have drove the illithids out into the open. So, how many illithids are there, population-wise, compared to humans and whatever human-like/humanoid races you want. You throwing in elves, dwarves, gnomes and what not in there? Or are you going with strictly psionic races?
Perhaps there were great wars between the two. Psionics and non-psionics, and the non-psionics won. Humans are now slaves? Pockets of resistance? Elves turn to magick to help fight their psionic masters? How are, and why are the illithids in charge? Now if there were powerful enough to put out the sun, they could have easily orchestrated the domination of their enemy races.
Perhaps the illithids are the secret masters of the world. That no one knows that they are the ones who caused the apocalypse. Sort of like the illuminati. Are you familiar with Zecharia Sitchin, and his 12th Planet theory?
Dim it? Explode it? Move it further away? -- Perhaps they might know that the planet moves around the sun, and they just figured out with magick to move the planet further in it's orbit. While everyone else thinks that the sun moves around the planet.
So, upon reading your post...
Perhaps the Aboleths had found means to perhaps access great vast underground oceans or fissures that had been created when the planet cracked itself and the oceans flooded these networks of newly created places. Or the Aboleths had figured out how to flood regions of the underdark/underworld/under-regions. Whatever you want to call it.
Once the sun returns to the sky, this would again create great geographical changes. Major flooding, which goes in line with Aboleth rebellion/conquest. There is a geological theory that vast flooding of the planet occurred not because of some great astrological disaster. But because of slow gradual climate change. Scientists first believed that when we left the ice-age, eventually the glaciers would melt away. But this new theory says that they did some receding, but for the most part, they also melted, building up vast ice-locked lakes. Then, because of the massive weight, eventually they all fail, generally all around the same time. Like massive ice-dams all over equatorial regions of our planet. Thus all the early cultures ALL had flood myths because everyone was flooded. Ahh, seems Turanil is familiar with it.
This theory also has a lot to do with the geographical region 200 miles east of Seattle called the Badlands.
My, a lot of people are thinking of the same things I did. Just goes to prove there are no true original ideas anymore. Geothermal mists... I forgot about those.