Hel, Queen of Hell
One of the foul creations from the coupling of Loki and the giant Angrboda, Hel was the ruler of the netherworld (Niflheim or Helheim) where men who died out of battle went.
Her realm also had a place for criminals, oath-breakers and assassins, all taboo in Nordic society.
Hel was pictured with upper body of a beautiful maiden and the lower body of a rotting corpse.
The darkest of the entire Nordic pantheon, it is unlikely that she was worshiped, but she is associated with dark magic and revenge. To invoke her was thought to be very dangerous indeed.