The Miri Nigri are mute semi-human dwarves. They are created by Chaugnar Faugn from the flesh of toads. Their normal behaviour is to serve Chaugnar Faugn without question or hesitation, and in silence. They are usually found crawling over the Great Old One's body.
The role-playing supplement Malleus Monstrorum expands on this information to suggest that twice a year they descend from the hills to abduct human victims on whose blood their master can feast.