It is a temporary room for certain parties/events in Club Penguin Rewritten. It may or may not return in the future.
The Graveyard was a party room in Club Penguin Rewritten that was accessible during the Halloween Party 2018. One of the keys needed to open the trunk is hidden in this room, which is inside the grave with the key symbol. After clicking it, a flipper would appear, with a key being held by it.