Creating local fog/transition into empty space

Hello everyone,

I’m creating a model for a cave and looking for ways to mark passages that lead to unknown areas. I’d like the hallways or passages to fade out into darkness or an effect similar to this, but having troubles finding a solution that is viable and doesn’t take hours of work to implement. For reference:

The circled passage is an example of one I’d like to fade or somehow transition into darkness. I’ve tried creating gradient textures but that takes an overwhelming amount of work to manually implement for each passage. I tried a fog tool or model that resembles fog that I can paste at the end but having no luck finding one. I tried the fog tool but of course it’s not specific enough to be applied to specific areas of the map. I even tried to import a bushy-tree model and paint it black so it serves as a noisy gradient but that didn’t work either!

Any suggestions are greatly welcome, spend hours on this and have no luck finding a solution.

Maybe you could paint the walls with a gradient material as suggested for light cones in this thread:

Here I placed an image with a radial gradient from black to transparent across a “corridor”:

Thank you for the reply, that sounds like it might be useful! Is there any place where I can find a tutorial on how to perform a radial gradient? I’m only familiar with placing gradients on regular flat square planes.