|A nearly full gloom lamp, used to harvest unlight.|
You can't see unlight under normal circumstances. It just isn't concentrated enough for the eye to detect it. Most adventurers only see it when somebody casts a darkness spell. Even that involves only a minute amount of unlight, just enough to repel the photons in the area affected. More concentrated motes of unlight will drift about like water in zero gravity, unaffected by wind, breaking apart into wisps of inky darkness. Any solid matter that touches unlight is stained black and oxidated, resulting in the destruction or at least ruin of most nonmagical objects.
Sources Of Unlight
Unlight seems to exist nearly everywhere, at least in very small amounts. The few intrepid wizards that know about the energy have detected the strange emanations under every circumstance imaginable. Some suspect that unlight is an unknown form of the negative energy that provides motive force to undead, or at least related to it. No experiment has yet proven this.
The more concentrated forms of unlight are occasionally found in nature: adult phase spider venom contains a high concentration. Certain wraiths seem to leave a trail of it and emit a particularly large burst when slain. I suppose a sufficiently dedicated wizard could farm wraiths by feeding people to them (wraiths usually transform their victims into more of the same). I can't imagine that having a happy ending.
If your campaign has some sort of primordial creature from the outer darkness that existed before creation, it's likely suffused with or even the source of unlight. It may have left behind nodes of the stuff in temples or forgotten places.
If you talk to the right djinn, it can explain how to make a a lead-lined "gloom lamp" that will slowly accumulate unlight over time. A night hag could probably sell you a canister of the stuff, as could the Ishnindah.
|A drear clock keeps perfect time.|
Okay, there's this weird energy that's kind of everywhere. But what can you do with it? Aside from transform fungal or arachnid creatures into true horrors. Well, there's always magic items.
If you bathe a suit of armor (leather, metal, whatever) in unlight enough times (alternated with mending spells to keep the suit from being destroyed), it will become the color of squid ink and feel noticeably chilled to the touch. Such armor renders the wearer invisible (as the spell) to creatures more than 30 feet away so long as one remains motionless.
If you can stabilize liquescent unlight (not too difficult for a spellcaster of 5th level or higher), it is possible to transfuse it into the inert circulatory system of a zombie. Such a zombie has resistance against fire (half damage), +2 to their speed, and is invisible when more than 30 feet away from a creature. The zombie will dissolve after a month or two.
One can craft a remarkably accurate clock powered by a leaden chamber full of unlight. Such a clock keeps perfect time (rare in a medieval society) and only needs refueling once a century. For reasons that are unclear, the clock enrages any good-aligned outsiders that notice it. Such a creature must succeed at a Will save or immediately seek to destroy it. The clock doesn't radiate evil or anything, the outsiders seemingly can't help or explain themselves. More usefully, divination is difficult near the clock. Divination spells of 5th level or lower cast within 100 feet of the clock or targeting within that vicinity simply fail to function.
A suitable refractory stone (spinels work well) that bathes in the interior of a gloom lamp for a few days will transform into a glittery black stone that seems to shimmer constantly. It's difficult to look at and trying for too long will give you a headache. Aside from jewelry, wizards like to crush gloomstones into gloom dust and snort it. It doubles the range of all darkness-related and shadow-related spells. Spinels are kind of expensive (they are probably considered rubies in your game world), so it probably consumes about 50gp of dust per casting, assuming you already have access to unlight.
Any or all of the issues below might affect characters handling or exposed to unlight.
•Creatures that are exposed to unlight lose a day off their natural lifespan per day exposed to the stuff. In essence, you age twice as fast. Their bodies do not visible age, they just stop early.
•Overfull gloom lamps have a nasty habit of going critical. If you forget to empty one for more than three weeks, it will sit there fuming out little tendrils of darkness until somebody actually touches it. Then it will explode, sending shards of lamp everywhere (3d6 damage, Reflex half), 2d6 cold damage, and causing them to suffer the effects of a nightmare spell as though it were cast on them each night, for 1d8 nights. During this period, the player also ceases healing naturally.
•A creature foolish enough to consume unlight will die a terrible death. The immediate effects will include terrible chills radiating throughout their circulatory system and blindness (the eyes turn entirely black, and may dissolve completely). After a day or two, the body will boil off into strands of cobweb.
•Touching concentrated unlight will stain skin for a year and a day. If wizards were more involved in the criminal justice system, I could see this being a decent way to mark criminals.