Here is a monster idea stolen from Dr Who, the weeping Angels...
The
Weeping Angels are a group of hunters featured in the
Tenth Doctor episode "
Blink". Because their physiology is quantum-locked, they only occupy a single position in space when seen by an observer (see
Schrödinger's Cat). When they are not observed they become a "quantum wave form" that occupies many positions in space, thus they cannot move while being observed; but when they are not they can appear to travel exceedingly quickly. They use this ability to approach and attack unwary prey. They turn to stone when observed, acting as a defense mechanism, because according to the Doctor, "you can't kill a stone." While in their locked state they appear as stone statues, often covering their eyes so that they will not see each other and lock themselves forever in stone form. This defense mechanism is what gave them the name "Weeping Angels".
According to the Doctor, "[the Weeping Angels] are as old as the universe (or very nearly), but no one really knows where they come from." He also describes them as "creatures of the abstract", "the lonely assassins", and "the only psychopaths in the universe to kill you nicely", because their method of killing doesn't do anything of the sort: a touch sends their victims into the past to live out their lives before they were even born; the Angels then feed on the "potential energy" of the lives their victims would have lived in the present.
These things look like winged angels with nasty fangs, obviously the whole sending you back in time thing wouldn't work but maybe they could drain levels or something... Honestly the episode Blink was one of the scariest TV episodes I have ever seen, (I highly recommend watching it on Youtube or something) it won a Hugo and Weeping Angels were voted the #1 scariest monsters of 2007... I thought they would make great monsters for D&D... Perhaps they could extinguish light sources, be invulnerable in stone form, make it so the players have to use their heads a little more than their axes... I don't know... Wanna give em a try??? I was thinking around Level 12ish or so...
"The Lonely Assassins, that's what 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 are 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 into 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. Then, oh yes, it can." The Doctor