The second of the three races of Beshaki, the Umbra live in the twilight third. This is the central third, trapping the Umbra between the night third and day third, which hasn't been good for the Umbra. First they had to deal with the Lumen trying to take over their territory, by the Lumen Knights, and not long after the Lumen gave up they found themselves under attack from the Arctus. There are still problems between the three races, so the Umbra work hard on keeping their borders safe.
Twilight magic, the power of the Umbra, is something that only the walkers and the Umbra know a lot about. It's a very different magic to many of the others because it gives them an ability to work with both the dark and the light. This is part of the reason both the Lumen and the Arctus tried to destroy the Umbra, believing that they could easily take over Beshaki and use their magic in all of the three parts. What they don't understand in the limitations of twilight magic, which are more limiting than those of day or night magic.