The Arctus, the race of night, are the third race of Beshaki and live in the final section, where it is permanently night. When they were created they were given night magic, the opposite of the Lumen day magic, which the Arctus keep secret in the same way the other two races do, although they know that the Walkers, whose knowledge covers the whole web, have a basic understanding of what night magic can do.
Unlike the Lumen the Arctus' war on the Umbra started due to fear of what the race could do, because twilight magic, the magic between night and day, was seen as better than either and much more dangerous. The war ended when a new barrier was created between the Arctus and the Umbra, but no one knows who exactly created it. Most of the walkers believe it was a demon, getting involved in things they shouldn't, while both the Umbra and the Arctus believed it was created by the other race.