I have another idea for an adventure brewing, although currently it’s pretty early days.
I was thinking about the caverns in the Elder Scrolls games, and how they’re never entirely pitch black despite being deep underground. There’s always a useful beam of light or small, high gap in a ceiling – never mid the inhabited caves that have torches and sconces.
My idea for the twilight caves is to have them filled with a magical twilight that means creatures can see, but everyone sees in black and white and shades of grey, regardless of darkvision abilities (except warlocks with the Devil’s Sight ability, because warlocks).
So far I’m thinking some kind of altar consecrated to twilight, maybe fill the place with shades and shadows, and normal cave creatures like darkmantles and ropers. I might try and develop a twilight dragon but that seems like a lot of work – I might just use some kind of evil or undead creature (or maybe a fey?) some features from the twilight cleric domain.
Hopefully having the location be caverns means they can be dropped into multiple settings with little adjustment, and can benefit more groups to make their adventures there.
Now I just need some plot hooks to drop in, and maybe a map…