(Why yes, I am stretching the ‘evil spirits’ theme just a bit.)
They are the curses sworn by a victim against his murderer.
They appear as horrid winged crones with serpents wrapped about their arms. Seeking vengeance for the wrongs of men, if these Erinyes or furies are after you, your death will be horrid and painful. By the time you see them, it is probably too late – unless you can offer some alternate task or sacrifice to placate their wrath.
Image – The Remorse of Orestes on Wikimedia Commons