Encroaching Darkness is a Chiurgical upgrade in Torment: Tides of Numenera.
This tiny, ink-black blot is a living creature, made of thick fluid. When applied to skin, it can shape itself into a number of different kinds of black tattoos, shifting easily into complex glyph shapes. When the wearer is asleep, the Dark crawls over the skin, changing into new patterns as it finds a new place on the body to rest. When it has found a suitable spot, its tendrils dig deep into the flesh, providing some form of a restorative boon.
The exact nature and origin of the Encroaching Darkness is hard to determine. Aeon Priests have debated whether an individual Darkness is in fact one entity or an entire civilization living on another being's skin. The relation of the Darkness to the wearer is meant to be symbiotic, but the symbiosis appears imperfect.