For an upcoming Veins of the Earth session of mine, I invented the Lightthieves. They will not make much sense for those used to other OSR games, as the light sources there do not have a “Strength”. But the rule of the Veins in that regard is simple: the Strength of a light source is equal to the number of feet it illuminates, and the other way around.
“You do not see them. It does not matter if you can see in the dark or not, how bright your light is or not. Even if you could see ghosts and invisible things, or detect movement around you, you would not notice them. They are just to tiny. The dot-like flies around overripe fruit look humongous compared to them. They are like dust particles, and rest with them when inactive. When you pass by, you stir them up along with the dust and they follow you for the light you carry. Light brings them back to life. Fleeting life. You will only notice that your light grows dim. Likely not right at the start, as this takes a while…
The light loses a point of Strength in the first hour, then two points in the second hour, three in the third and so on. At some point the PC will notice it, but it is unlikely that they know what the reason is. They may think that their light is failing, and just light another one. This will not change a thing, of course, as the light is just leeched away by the Lightthieves, and they multiply while they do.
Only when the Strength of light becomes 5 or less will the Lightthieves be noticed, as small dots that are barely brighter than the surrounding. They drift around the PC and their light source. When there is no more light, they become visible: dozens of them (2d6+2 for every point of Strength they have leeched from the light source). Little glowing particles, just like dust-sized fireflies. They slowly circle around and glow with a light Strength of 6, but will stop doing so after a few minutes. Then, they will disperse and lazily drift apart from another. Some will sink to the floor. After a couple of hours, they will all stop drifting and start sinking. Their glow dies and they become inactive again. That is, unless another light shines on them.
Dispel Magic will render the Lighthieves inactive for a turn (and they will not rise again if no light shines on them then). Protection from Evil will merely force them to keep a distance. If this does not force them to leave the shine of the light, they will still leech it away, hour by hour, and grow in number.