dnd5e-helpers VS perfect-vision

You didn't state what system you are using, but if it's 5e, and if you don't mind a visual indicator that is not tied to CUB then checkout the module DnD5e Helpers. The section for Combat Action Management is what you'd want to look at. This shows a visual indicator for Action, Bonus Action and Reaction.

DnD5e Helpers: General helpers around 5e (eg. Regeneration prompts)

Use this module https://github.com/trioderegion/dnd5e-helpers

midiqol, there are also modules for cover, for eg dnd5e-helpers

This is one of the things the module Dnd5e Helpers helps automate. It is triggered when an actor with a feature called Undead Fortitude drops to 0 hp. It prompts you for the damage type, and gives you a button to roll their Con save, and healing them on a success. It's configurable too.

DnD5e-Helpers for managing the Regeneration of vampires as well as resetting legendary actions of legendary monsters.

in foundry v10 you can do that with the module perfect vision which can set up areas where global illumination is disabled, alas some change in foundry v11 make updating the module too complicated for the author so he abandoned the project

You can use the module Perfect Vision to draw areas of darkness, limited vision etc.

Perfect Vision v4.1.14 Bugs URL

Look no further, perfect vision does exactly what you want to do: https://github.com/dev7355608/perfect-vision

Have you checked out Perfect Vision?

Perfect Vision v4.0.17 [Bugs URL](https://github.com/dev7355608/perfect-vision/issues)

perfect vision lets you do this with their vision limit setting in a drawn shape

Clicking on a drawing you should see a lighting menu like this https://github.com/dev7355608/perfect-vision/blob/main/images/drawing-config.png

Alright, this isn't a direct answer to your question, but more of me trying to answer what I think your problem is. I believe the solution is using https://github.com/dev7355608/perfect-vision

Perfect Vision: Improved vision support