Tonberries are vicious creatures that dwell in the dank caves and grottos of the Underdark. Hostile and xenophobic in the extreme, they wander the lightless passageways of their subterranean homes in search of intruders to kill. Subterranean Skulkers. A tonberry is a small, humanoid amphibian with green skin, large yellow eyes that lack visible pupils, and a fishlike tail. They dress themselves in heavy brown robes and are rarely seen without a lantern in one hand and a wicked knife in the other.