The Grosche were a violent and carnivorous species with many teeth. They used soft-light disguises to walk amongst other races.
Ilmatar used sympathetic vibrations to burn his audiences and feed the corpses to the Grosche. This was the plan at Cardiff Bay Millennium Hall, but Ilmatar only killed Sigmund Wallfisch. Before then, they ate a Pansian Eargrub and tried to kill Toshiko Sato and Ianto Jones. (AUDIO: Dinner and a Show)