And not just any graveyard....In my mind unicorns just like elephants have a ritual of death. They have their sacred grounds where they choose to be at the last moments of their lives.
This graveyard is not just for unicorns, though. Other mythical horned creatures come there too. The skull laying over there is from a creature called Mammodox, which is more or less a crossing between a rhino and a mammoth. (I'm working om more detailed of that species).