Darkreach is a habitation founded by Alicorns immediately over the edge to the dark side of the world. It was planned by Onyxxus and intended to be a launching point for expeditions into the dark side, with the purpose being to retrieve the Harmonic Prism.
The base is built inside of a mesa surrounded by a forked canyon. Two bridges connect the mesa on the top level, and a large surface-side cargo hatch allows access. The facility itself is several layers of workshops, living quarters, and machinery to generate power and circulate air.
When the Herald finds Darkreach in Ofolrodi, it is entirely powered down and uninhabited. Curiously, it did not contain bodies, nor did it appear to be ransacked. The only immediate danger was a single changeling egg in the basement. With Flynn and Kepler's work they were able to power the settlement back up and gather much-needed supplies and intelligence from what was left behind by the previous expedition.