The Night is the "power" generated by belief and sacrifice to the twin goddesses Sve Noc. This power is mostly used and harnessed by the Drow of the Everdark as well as Catherine Foundling, who was granted the title of First Under the Night by Sve Noc after striking a deal with them following the taking of Winter by Sve Noc.
Night Is contained within its user. The drow take Night from others both living and dead. When taking from the dead it leaves bloody holes as it leaves them.
If a Drow's corpse is left unharvested the Night will fade
By killing people, even those without Night, the Drow can grow the Night.
Secrets are skills and knowledge a person has that can be harvested along with Night. Secrets can be mundane skills or ways of using Night
The foundational virtue of Night is to prove yourself worthy to rise.
The Night is also synonymous for the religion of the Drow.
The use of Night enables the unusually long life-span of the Drow as well as their powers.
With the return of Andronike and Komena's godhead, and the achievement of becoming gods, Night is no longer restricted to only the Drow. The first non-Drow practioners capable of wielding Night are two humans, two orcs, and two goblins.
One of the goblins was capable of using of blackflame while one of the orcs had enhanced nightvision. Catherine also judged that the Night provided an extended lifespan to their wielders.