The morrok was a monster from Drayan stories that was said to come at night and take people when they died. It was said to live in a cave on Crysata, and that it could not be seen until you were about to die and it was too late.
In 2372, a group of seemingly young (actually elderly) Drayans on the crysata moon, whose attendants had died in an accident, told Tuvok that the morrok was responsible for the disappearances of several of their number. In fact, they were dying of natural causes. (VOY: "Innocence")