Whilst at first glance this may appear to be an alien creature, or a monster from the darkest corners of your imagination, it is in fact a simple hermit crab who's made an unusual choice of home. Hermit crabs live inside shells but aren't born with their own. That means they have to scour the environment around them and make use of whatever they come across.
As the crabs grow, they need larger and larger shells in order to remain comfortable. A human shell is a good size for a hermit crab, and so this one has probably settled down for life; unless it comes across another hermit crab. Because shell space comes at a premium, the crabs will do battle over prime real estate, with the winner walking away with the prize. The real question here, though, is whose skull is that, and why is nobody investigating it?