This spell darkens the room you’re in somewhat and lasts about 1 minute per player level. It lasts a long time, but if you need to use this spell regularly the best practice is to either carry some nullium nuggets or have some of your gear permanently ‘dimmed’ by an alchemist.
The ingame help reads:
No saving throw. Casting time: 0 rounds. Type of spell: neutral spell. Affecting stats: wis. Cast type: control. Spell point cost: 4. While most depend on the light of day in order to be able to conduct their every day activities, there are some who shun the light, either for reasons of necessity, or preference. By chanting a few words, they can bring forth a sphere of darkness that surrounds them. The more powerful the caster, the more enveloping the darkness they bring forth. To dismiss the darkness, just touch it.