The Fiends are unseen entities in the final episode of the Hood series. They are first mentioned by Merappi when talking to the Wolf. He mentions that he was appointed to guard them after being unable to keep his end of a bargain made with a demon. He warns the Wolf that the only way to stop the Archdemon is to release the Fiends into the world and cause havoc. When the Wolf places six blue gems on the lock nearby, both cells open, releasing the Fiends into the world and allowing the Wolf to confront the Archdemon.
During the events of "Kingdom of Liars", the Fiends are mentioned once again in a cultist's diary and they plan to use them.
In Kingdom of Liars: Stonepath.
Cameron, Baldewin and others set out to face a fiend who has surfaced. The new hero of the Hernessian guard solves the secrets and riddles in Stonepath that protect the Fiend. It's eventually Cameron who has to face the Fiend, but she has been trained well by Baldewin. With some effort she quickly disposes the evil red creature.