Furies

Fury, plural Furies, are creatures on ABC's Once Upon a Time. They originate from the Underworld, to collect unpaid debts for the price of magic.

Before Fourth Curse
Emma heals Robin Hood with her dark magic by Regina's request, but at the deepest price, meaning a Fury will come to pay the price of magic.

After Fourth Curse
A fury does indeed go after Robin to pay the debt after Regina was sent to pay for it in Camelot. She gives her life up for Robin, but is joined in by everyone, who now believe that Regina is a savior indeed.

Trivia

 * They are based on the furies from Greek Mythology.
 * Since they come from the Underworld, it is unclear whether or not they can be permanently destroyed.
 * Furies only come up from the underworld when the moon reaches it's zenith.

Appearances

 * S5, E02: "The Price"