As part of its ongoing efforts to promote every single Pokémon out there, The Pokémon Company has shared special content dedicated to Umbreon:
Did you know? When exposed to the moon’s aura, the rings on its body glow faintly and it gains a mysterious power. When darkness falls, the rings on the body begin to glow, striking fear in the hearts of anyone nearby.
Source: @Pokemon
Source:Pokémon