A small insect, not dissimilar to a firefly, which, as a defense mechanism, is able to warp waves of light, magnifying it and intensifying. It increases the ‘Lux’ value substantially, and, using these extremely bright flashes of light, is able to confuse, distract or even blind both its predators to make a hasty getaway, or to confuse its prey (smaller insects).
These creatures are mostly nocturnal, but they will operate in daylight hours. If trapped in full sun, they will glow so brightly they "burn themselves up".
Now, If an adventurer were to catch several of these and place them in a transparent vial or bottle - He now has an inexhaustible, efficient light source when traversing those dark dungeons. The insects will increase the light value substantially, so even the dimmest of rooms will be lit up - of course, it wont work in pitch black, as the insects would need SOMETHING to work off.
Now, this would also be useful in brighter areas: A goblin swings a sword at an ally, you flash them with these insects, the goblin gets blinded or distracted, thus missing its foe, and hitting another goblin instead.