Fear of the dark,
Because why not
After 15-30 minutes without light your character is getting paranoid, fear builds up dealing new wound that reduces agi, int, str, maybe Buffs survival and will, as he panicly wants to stay alive
That would enforce fireplaces in mine, possibly reduce botting, surely piss off many players, and troll custom client users ( I use Ard's client and I love features but still, I would like that add-on to the game)
Thoughts?