Each time you or another player need to add at least 1 ghost to the mist, you must also add 1 ghost to 1 open cemetery on the board. A cemetery is open as long as at least 1 of its 4 sides is still open.
If there are multiple open cemeteries, you can decide as a team where to add the extra ghost.
If there aren’t any open cemeteries, then you don’t need to add any extra ghosts.
You cannot add a meeple to the cemetery itself. You may, however, place one onto a clear road or city on the tile, as per usual.
You place a tile and must add 2 ghosts to it. Unfortunately, the cemetery isn‘t closed yet, which means you must also add 1 ghost to it. Finally, you place a meeple onto the city.