Nis is the name of the god as well as the land of sleep in which he dwells.
People see him as the god of wisdom and divine inspiration and so he is often invoked when someone experiences an epiphany. Due to this he is said to have inspired Arthur Eld to unite Mid-World as well as the Commala songs and dances. If a man ever falls into a coma he is said to be stranded in the Land of Nis and so he is invoked to help the lost soul into returning to his physical body.
It should be noted people do not blame Nis if this happens to someone; it is merely that a mortal's mind can become easily trapped within Nis' extra-dimensional realm.
Connection to the Crimson King
The Crimson King names his horse Nis after the god, hoping that it would allow him to ride into the minds of sleeping humans to invade their thoughts.