The Resting (Mob)

This very rare mob would spawn only from super rare gravestones. The gravestones can spawn on any naturally occurring block that makes sense (Dirt, Sand Gravel). It is a ghostly type mob that rise from the grave whenever you got into a 5 block radius. It would have a skeleton face and wear a faded cloak around itself. This mob would be untouchable as it's ghostly figure makes it almost impossible to hurt. When the Resting spawn, a cloud of fog will surround you. The best way to get away from it would be to run far or get out of it's sight by hiding. Splash potions and water can hurt it, but the mob is smart and would most of the time dodge these attacks. It doesnt stand much of a chance against lingering potions though. Potions of invisibility would trick the rested into thinking you've gone. You can then head back to the grave and collect special loot or items from it. Also, the grave wouldn't be a structure. It would be a naturally spawning and despawing thing so graves could still show up even if you've already explored the area.