The Fossegrim lives in waterfalls (hence his name), and are widely known as the best fiddle-players. Ever. A Fossegrim can play so well that none who hear them can refrain from dancing. Not only do they posess nearly supernatural skill with the violin, but they can also teach this to mortals. This requires some skill beforehand, and many months of vigorous training with the Fossegrim. The last step in the apprenticeship is when the Fossegrim grips the pinky-finger of the mortal and twists it all the way around. This enables him to move the finger in all sorts of ways that he couldn't before, thereby allowing much better control of the strings.
Mrriv house in particular prize Fossegrims, for in addition to being good musicians they also make the population at large more musical in time. They are often invited to festivals and dances, though some of the more dignifiet effetes are considered a bit above their lively style.