Light and shadow

Being unilluminated by light does not always make you difficult to see: Even the darkest shadow is unhelpful when your sillhouette is on a backdrop of brilliant light. That said, a creature hidden in darkness against a dark wall should certainly get the concealment benefits of shadows.

Various items cast light out to different distances. Stacking light sources progresses like the cube root; 8 torches illuminate an area twice as wide as one. Bright illumination is followed by the same distance of dim illumination.

Torch:

Torches brightly illuminate out to 5 meters.

Lightning Flash:

Lightning very briefly illuminates an area out to 18 meters.

Sunlight:

Sunlight illuminates all it touches, and those things reflect enough light to illuminate out to 10 meters.