Monster of the Day: The Lonely Assassin

Source:  Dr. Who
Location:  In particular a lonely old house in England.  Theoretically anywhere in space and time.
Threat Assessment:  6.  They look like statues of weeping angels.  They are not, they are nasty lifeforms that will if they catch you send you decades into the past.  That's the nicest thing they do.  Oh, and they do that so they can feast on the unused potential of what was your future.  The worst thing is they only move when you aren't looking at them.  Look at 'em and they are a statue.  Blink and you are dead.
Limitations:  They can't even look at each other.  They are truly lonely.