Ultravision

Ultravision was a type of vision possessed by many creatures in the Realms that allowed them to see in some instances of darkness.

Nature of Ultravision
Ultravision allowed a creature to see radiation in the ultraviolet spectrum, above what was possible by normal eyesight. This included gamma rays, X-rays, radio waves, and more.

Creatures that possessed ultravision could see in normal outdoor, night time darkness as if they were in twilight. This was because Toril and many other planets were continually bombarded by ultraviolet radiation from stars and other celestial objects. In these conditions, creatures could see clearly out to. Beyond that and up to away a creature could see things only dimly. On particularly cloudy nights, this visible range was reduced by half.

As earth and stone screened out that radiation, this vision was of little use inside subterranean locations unless there were some form of ultravisual emanation. Ultravision also did not function within the Inner Planes, but would function in the Ethereal Plane and the Astral Plane.

When underwater, the visible range for a creature with ultravision was typically half that of their land range. At ocean depths below nothing could be seen, for ultraviolet light did not penetrate beyond that depth in sufficient quantities.

Spells and enchanted weapons that shed illumination typically would hamper this vision.

Trivia

 * There were a variety of magical items that could bestow ultravision upon a creature. These included the Eye of Vecna, the robe of eyes, the lens of ultravision, the staff of night, the magemask, and the catstaff.