the lonely assassins, they used to be called. no one quite knows where they came from, but they're as old as the universe, or very nearly, and they have survived this long because they have the most perfect defence system ever evolved. they are quantum-locked. they don't exist when they're being observed. the moment they are seen by any other living creature, they freeze into rock. no choice. it's a fact of their biology. in the sight of any living thing, they literally turn to stone. and you can't kill a stone. of course, a stone can't kill you either. but then you turn your head away, then you blink, and oh yes it can.