(Inspired by the Darkest Dungeon, a game that is way too good.)

Introduction

Torches and lanterns are a staple of fantasy roleplaying games. They may have grown a bit stale, however. Recently I've been playing Darkest Dungeon again, and an idea dawned on me - why not use the Light system from this game in TTRPGs. The following system assumes:

• 2-in-6 basic chance of surprise on both sides
• Crits on nat 20
• That most species don't have darkvision

Light Level

Your party's Light Level is dictated by how many members of your party are carrying light sources + the ambient Light Level of a location. Use your party's as long as they remain together (no specific range is provided, use your better judgement) - if they split, then use the ambient light level. The following bonuses and maluses assume that the party(and allies) are outsiders to the dungeon, while the enemies are the dungeon's residents. Flip if needed.

Light Levels

Radiant (4)

Outside during day, or in a well-lit room.
+1 to AC for the Party(negate if both sides are used to light), 2-in-6 chance of an omen of a nearby location with each turn, +1 to the chance of surprising enemies(negate if they are used to light, it represents not actual surprise but them being blinded by light), -1 to the chance of the party being surprised.

Bright (3)

Inside during day with no extra light sources, dawn, dusk or a storm outside.
1-in-6 chance of an omen of a nearby location with each turn, -1 to the chance of the party being surprised, +1 to stealth checks

Dim (2)

A clear night, or inside with an unsteady or dim light source.
+2 to all stealth checks, +1 to enemy crit range, +1 to enemy hit and damage.

Gloomy (1)

A cloudy night, or a single candle in the infernal blackness of a cavern. This is the minimal light level as long as even a single light source is in a room, even if unhandled by Man
+2 to all stealth checks, +1 to party crit range, +2 to enemy crit range, +1 to enemy hit and damage, -1 chance of the enemies being surprised.

Egyptian Darkness (0)

A completely overcast moonless night, or your average dungeon.
+3 to all stealth checks, +2 to party crit range, +3 to enemy crit range, +1 to enemy hit and damage, -1 chance of the enemies being surprised, +1 chance of the party being surprised, -1 to allied morale. Ranged attacks are impossible unless you have other ways of knowing where your enemy is. There is also a temptation, created by the forces of Darkness - all treasure found in its complete domain is multiplied by 2 (extra coins, with faces that have too many eyes, or gems in colours that are not in the normal rainbow, artworks that depict forbidden gods - use your imagination).

Designer's Intent

Tracking light is sometimes annoying. I've tried to make this system as a way to incentivize (Luke please don't kill me) the players to track it - by providing both bonuses as well as the usual maluses, some of them quite useful, I hope that it will make the players want to take more care of their light. As more and more light sources die down (Target the torchbearers! The monsters aren't stupid!) the tension rises, and so do the chances of critical hits - both allied and enemy. Finally, if the team decides to go in complete darkness, the game will be full of thrills and great treasures - but will also be full of death.