One especially dramatic scenario would be for all plants to die out, resulting in a world populated entirely be 100% carnivorous animals and people eating each other, with a very rapidly dwindling population. Combining this "red in tooth and claw" ecosystem with the perpetual darkness would lead to a terrifying mood! In absolute darkness the population would dwindle way too quickly though, but it could be tempered to have an arbitrarily slow population decay rate by adjusting the amount of light the world receives (and thus the rate of extinction for plants).
Another option would be to explore alternative food webs. In the real world, total darkness still allows a few types of ecosystem. There are oceanic plumes which support a food web on chemical energy captured from the plumes by bacteria. In your world I would imagine there are highly magical point sources which emit "life energy", powering small island ecosystems around themselves.
The rest of the deep ocean runs entirely on food particles which drift down from the upper reaches (it's called oceanic snow). Filter feeders scoop this up and are eaten by predators. If the darkness in your world is caused by a cloud layer, then there could be some kind of flying or floating plants existing above the cloud deck, powering an ecosystem on the ground with discarded, rotting leaves. Detritivores like earth worms and millipedes would feed upon the leaf mold. Larger animals would eat the detritivores and mushrooms. People would eat everything. This strikes me as another very dramatic option!
Cave ecosystems largely run on debris brought in by animals like bats, which leave the caves to feed. Copraphages and detritivores like cockroaches feed off the excrement of the bats (and fungus that grows on it). Check out this piece by the amazing David Attenborough https://www.youtube.com/watch?v=UC51eymvsRA Your world could conceivably have aerial access to another realm where the sun DOES shine. Bats, birds, or flying insects might feed in the sunlit realm, then return to the realm of darkness to roost. The sunlit realm might be very tall plateaus which rise above the opaque cloud deck. For example, see the amazing tepuis https://www.youtube.com/watch?v=S9K8QcjwjYs
The last real world ecosystem I can think of are the microorganisms that live in the bedrock of the crustal plates. They have extremely slow metabolisms which are powered by chemical weathering of rocks and chemical reactions caused by the heat of the mantle. This ecosystem exists miles down into the earth. It's hard to see how your story could revolve around something like this, but maybe you see a possibility that I've missed?
Anyway, my advice is to figure out how your ecosystem is powered first. Everything else (technology, culture, ecology, etc.) will follow from that.