Dragon Graveyard is an Advanced Cleric Trap Room introduced in Boss Monster: The Next Level
The nature of this card allows it to work well with other Cleric cards, which usually allow the player to take cards from the discard pile. This card works well with other cards the require the player to activate card effects by means of discarding from their hand, such as Dracolich Lair and Witch's Kitchen
It is unknown whether this is supposed to be the remains of a known dragon, or of a Dracolich. It might also, or might not, be a hint at where Dracolichs come from.