Mahr is a demon that feeds on its host's madness. To further feed itself, it'll cause the host discomfort and mental delusions to drive them further insane.
A mahr is a fox-like gremlin with six legs like that of a spider. As they tend to possess hosts, a mahr's physical appearance is almost never seen unless exorcised from the host. However, as this possession requires magic, using magical disruptive elements, like silver or meteorite, can force it out of the host, thus making the demon vulnerable to most attacks.[1]
One such being possessed the Wenck noblewoman in 1107 and was exorcised by Deglan when he ran into two of her child servants, Vesemir and Illyana, with blue gum and recognized the signs of a mahr.