get a deathberry from the medicine den in Fireclan territory, click a body of blue water (not the light blue ones) and you'll get a fish. you may have to wait until a full moon in order to steal the deathberries, or become a medicine cat by talking to Lunarclaw the medicine cat or Breezestar the Fireclan leader. You'll have to be a Fireclan cat in order to become a medicine cat, as Moss-shadow is still working on the game.
