Truesight

Truesight was a type of sense that only a small number of creatures possessed. Creatures with truesight could, out to a specific range, see in both normal and magical darkness. As well as that, they could see invisible creatures and objects and automatically see through visual illusions (to reveal the original form of a creature or shapeshifter).

Those with truesight could also see into the ethereal plane within the same range of their normal truesight abilities. Helmed horrors possessed this rare sense.

Spell
Some mages could cast true seeing, giving them temporary access to this sense.