They appear to be very dark, overly large worms - perhaps a handspan in length and half as thick as a finger. That and their matt, dull skin marks them as something other than a typical earthworm.
What do you mean, the little wriggly ones you throw at fish or the big rock ones that squish people?
Six months of hunting, six months of research and paying coin to scholars and sages, and now that we have the legendary wand of Geohadris the Stone King, it only summons stupid boulderkin? I am outraged beyond imagination!
Mapmaker's bane, a few of these can ruin a decade of cartography!
Magic should have side effects, both expected and unexpected - fortuitous and deplorable. Expected side effects add anticipation, while unexpected ones increase the drama of the scene. The result can be comedic or dire, it dpends on which circumstances work best for your game.