Shadow Legion's early-vassal. You'll possibly deal with this faction a lot if you're near the starting location of Shadow Legion, such as the nearby Bretonnia or High Elves. They'll probably have bunch of crappy stack of units, but usually, they can be found near Be'lakor himself, which can be very dangerous.
As Shadow Legion, this is your vassal. There isn't much to say other than it's probably not worth it to turn the Lords of this faction into a Daemon Prince since AIs aren't that good at taking care of their own armies.