The umbryn are large panther-like cats, ordinary beasts in every respect but one: shadow answers them. In darkness an umbryn cannot be seen and very nearly cannot be heard, and it can step into one shadow and out of another some distance away, crossing the gap through the dark itself.
This is a Vulus affinity laid over a plain animal. The cat works shadow as a substance, the way a dark-shaper pools and moves it, rather than merely hiding in an absence of light. Nothing about the umbryn is undying. It hunts, breeds, and ages as any predator does, and a killed umbryn dies the ordinary death: the three strands take their roads and the shadow leaves it with its last breath.