Cost per tile: 300
  Health per tile: 350

Monster Attracts:                     Size Needed:                       Also needs:
      Vampire                                    9 tiles                                   Lair

Once you build a graveyard, your Imps will drag any dead creatures in your dungeon to it, where the corpses will slowly decompose. There are two benefits to this: except for the Horned Reaper, all your creatures get annoyed when there are lots of dead bodies strewn around, so having somewhere to store them improves morale. The second and most important benefit is that once enough bodies decompose, a Vampire will rise from the grave.
 If bodies aren't decomposing quickly enough (or if your graveyard is full), throw in a Hell hound. He has a little habit that can help, which may be disrespectful, but it speeds up the process. Graveyards are not affected by their efficiency.