- "The dead envy the living, in time that envy turns to anger, even hate"
- — Lulu
In the world of Final Fantasy X, Fiend is a Spiran term for the monsters which inhabit the land. Fiends are the spirits of the dead who stay in the living world instead of departing to the Farplane, refusing to accept that their time has come. However, many of these ghosts are purposeless and unable to cope when their loved ones or they themselves move on, their sadness slowly turning into anger and then malice. At this point, the spirits use the Pyreflies to assume a warped physical form known as a Fiend. Once a Fiend, the spirit it formed from is driven to prey on the living. It was the task of the summoner to perform a Sending to guide the wayward souls to the Farplane before they can become fiends. If a fiend is vanquished, the spirit that held it together can be sent. If there is no summoner present to send it, the cycle will most likely repeat itself, leading to a virtually endless supply of fiends.
Not all Unsent are destined to be transformed into fiends, however. Some, such as Auron, Seymour Guado, and Maester Mika lived on after death because they had a purpose to serve; whether it was a promise to a friend, a desire to keep Spira's hope alive, or the desire to watch it die. Instead of using Pyreflies to attack the living, they instead use them to create a body identical to their old one, effectively gaining a second chance at life.
Fiends come in many different varieties, including wolves, birds, and plants. If a fiend is killed, pyreflies that form its body emerge and disperse the Fiend's physical form.