Gwyll (also known as Wendigo to some) are violent creatures that have shaped the human habitation of this setting. Their intelligence is untested, but they seem no smarter than normal animals. Before they knew what Gwyll were, the Goomba Corps coined the term "Grue" to refer to them.
Not much is known about the Gwyll. As a matter of fact, most people don't even know they exist. They will only be found during dawn and dusk, and where they spend their days and nights is unknown. People never leave town during dawn or dusk because of the Gwyll. Most don't know why; it's just been the rule for as long as they can remember.
The party met some Gwyll during the brief periods it has spent in the wilderness during Gwyll hours. They also met some Gwyll in the catacombs beneath Gretennan. Most of the Gwyll they saw were canine in appearance, but some were a little insectoid as well. Some were clothed and humanoid, wearing red head bands with Yin Yang patches sown into them.
It has been revealed that Gwyll can "turn" other creatures, though the method and effect are unknown.