Shedinja.

So, let's get this straight: If you stare into the crack in it's back, you'll lose your soul.

Go, SHEDINJA!

Total mind-fuck.

It should be noted that this is not a certainty. Yes, the PokeDex did acknowledge this, but it is implied to be a superstition. The Ruby entry mentions that "It is believed that this Pokémon will steal the spirit of anyone peering into its hollow body from its back." and in Platinum, it says that "Peering into the crack on its back is said to steal one's spirit."

That's it for now.