Nothing underground is lit. Not the corridor, not the room you are standing in, not the wall an arm's length in front of you — and the game does not quietly assume you brought a torch. A party that walks down with nothing burning sees exactly nothing, and finds out the hard way.
There are three you can buy, and they are deliberately not a ladder from cheap to good. Each one is the worst of the three at something, and choosing between them is choosing which currency you would rather spend.
A candle shows you the square you are standing in and its walls, and nothing beyond them — you can see where you are but never what is ahead. It costs almost no silver and burns for a very long time. What it costs is vision.
A torch is the first light that shows you somewhere you have not been yet: your square, and a dim glimpse of the next. It is the best light per coin in the game and it burns down fast — a fraction of the candle's life. And once lit, it cannot be put out. It burns until there is nothing left of it, whether or not you still need it.
A lantern is brighter again, and it is the only light in the game you can close. Shutter it and the fuel stops burning; open it later and the rest is still there. That shutter is the whole reason it costs many times what a torch does, and it is what makes it the cheapest light over a long expedition despite the price on the shelf.
Whoever holds the light holds it in their off hand — which is the hand that would otherwise have a shield in it. The party's light is therefore always one member's protection, and it is worth deciding on purpose who gives that up rather than letting it fall to whoever happened to buy the torch.
Only the brightest source anybody is carrying counts. Light does not add up: five lit torches illuminate exactly as much as one, and burn five times as fast. Light one.
Light is spent by time, and time passes when you do things. Every step down a corridor burns a little. So does standing still long enough. So does every turn spent at a camp, and every spell cast in the passage.
Turning on the spot is free, and so is walking into a wall — you can look around all you like.
A fight is the expensive one. An encounter charges its whole cost the moment it begins, before a blow is struck — a serious bite out of a torch, taken up front. A party whose light was nearly gone when something jumped them does not run out during the fight. They are already in the dark when it starts.
You are warned when a source is nearly finished, and told plainly when it dies, naming who was carrying it. Then the screen goes black, which is not a thing you can miss.
Being in the dark used to cost you only information. It does not any more. Every roll your characters make in a fight at no light is a die short — attacking, defending, casting, all of it, for the whole party, for the whole fight.
The things down there are not similarly inconvenienced. They live here. A dark room is a hazard aimed at you, not a leveller, and no amount of rage or resolve helps a character see.
What matters is only whether it is dark, never why. A chamber that eats torchlight and a torch that guttered out two corridors ago come to precisely the same thing, because they come to the same thing for the player looking at the screen.
The dark is survivable, which is the surprising part. A party with nothing lit can still walk, and the map still fills in — one wall at a time, as they walk into it. Every bump records where the stone was. It is slow, it is blind, and it works.
This is exactly the job the candle exists to do better: it shows you the walls of the square you occupy without your having to find them with your face. The torch is the first light that shows you anything you have not yet reached.
And some places simply refuse to be lit. Where the dungeon has decided a room is dark, it is dark, and the best lantern you own will not argue with it.
Light can also be conjured. It cannot be bought at any price, it costs mana rather than silver — which is to say it costs the caster spells they might have wanted later — and it burns nothing.
Two things make it worth the mana beyond brightness. It leaves the carrier's hands entirely free, so nobody gives up a shield for it. And because it is not an object, it cannot be dropped, spent, or left behind in a pack.