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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.

or, press c & m at the same time a type "deathberries" so you don't have to keep going into the medicine den to get the deathberries!