In the novel Lost Souls (the third book in the Star Trek: Destiny) the Borg are revealed to be the survivors of the Caeliar city Mantilis. Thrown across the galaxy in the Delta Quadrant and back in time to approximately 4500 BC by the destruction of Erigol at the climax of Gods of Night, the first book in the trilogy, a group of human survivors from the starship Columbia NX-02 and Caeliar scientists try to survive in a harsh arctic climate. Most of the human survivors die of exposure, while several Caeliar are absorbed into their race's gestalt to give life to the others in their group mind.
The Caeliar were the inhabitants of New Erigol, and formerly of the planet Erigol, which later became the Azure Nebula after the supernova of FGC-SR37-758. The term "Caeliar" can be used as a singular or plural noun as well as an adjective