The Umbral Choir is a race of eusocial dark matter beings that hide in scattered places across the Endless galaxy, plotting out the end of the galaxy's misery through subterfuge and manipulation.
The base biology of the Umbral Choir is an opaque gas that glows blue. When out of hiding, this substance holds the shape of a slug or humanoid. In hiding it simply resembles fumes.
The Umbral Choir appeared to originally exist as a single organism the size of an ordinary starship until it entered the rift to Coroz, home reality of the Riftborn. Attempting to make contact with an individual dying of that world's blight had fragmented it into multiple beings sharing a cognitive link.
The native environment of the Umbral Choir is the intergalactic medium, where it fed off of dark matter. While dark matter is their preferred sustenance, they can obtain energy from organic biospheres without difficulty.
Umbrals can augment themselves by absorbing an intelligent being to become an Umbral Shadow. These entities are a superior being in all regards at the cost of more ravenous energy demands.
The Umbral Choir have an inherent skill and capacity for manipulating electronics by mere touch. Very few races can begin to compete with their hacking expertise and many of their hacks are only noticed when it's far too late to investigate them.
The Umbral Choir hail from the void between galaxies. They are an old species, having remembered a time when the universe was quiet until they were drawn to the Endless Galaxy, hearing was a death cry. They believe themselves to be emissaries of bringing peace by any means necessary.
The center of the Umbral Choir's civilization is the Crescents, habitats built from a series of supercooled hydrogen rings to draw them from the "immaterial" plane into the physical world. Despite their extreme nature, these objects are potentially habitable to other races, although no other species exists inside the Umbral Choir's civilization in any significant numbers. The five crescents together hide around unusual astronomical objects as the Nexus, a star system onto itself. The Nexus can be instantly teleported to other astronomical phenomena through the potentially dangerous process of transmigration, where the system is implanted with the same sort of tetrahedral object that crescents are constructed from.
It is a common practice in the Endless Galaxy to subvert the devices of civilians for theft of resources and information, the Umbral Choir take it further by stealing people themselves. Billions of unlucky individuals can disappear overnight to be converted into Umbral Shadows. Even less lucky victims of this ability are painstakingly converted into pure economic growth through some horrific processes.
By transmitting a small Von Neumann probe into a star system they can construct Sanctuaries, lesser lairs that act as parasitic bases for their hacking operations and smuggling sites. A Sanctuary can occupy the same space, even the same infrastructure, as a more conventional civilization. These actions are all peaceful by their standards, and this peace is a two way street. Those who beat them at their own game hold the potential to all but exterminate them without ever declaring war.
- Endless Space 2: Penumbra (2019)