"They may be called 'pyreflies' but they aren't really 'flies', you see. They're those lights you see whenever a fiend dies. The little fellows are responsible for a few fantastic phenomena. Visions of the past, spheres, fiends—these are all the pyreflies' doing. In fact... pyreflies have something to do with aeons, too. The dreams of the fayth reach through the spirit of the summoner... And that which is unreal becomes real for all to see!"
Pyreflies are a natural phenomenon that appears throughout Spira. Pyreflies are inherently magical things, which appear to have some relation to life and death. When a being dies, small prismatic orbs with tails like comets will swirl up and move around them. They sometimes exist in areas for little apparent reason.
Pyreflies interact strangely with forms of spiritual corruption. Malevolence and Malice are both held at bay, but not purified, by concentrations of pyreflies.
Sending and the Unsent
Pyreflies help sustain life in Spira. The pyreflies are incapable of crossing Althena's Boundary, and will simply dissipate upon reaching it -- but as the Boundary stops a couple days' sail from the furthest islands of Spira, few have ever seen this. Those in Spira, upon death, will have pyreflies swirl up about their remains.
Summoners are capable of doing a ritual called the Sending, which directs the pyreflies back to the Farplane. If a Sending is not done, the pyreflies can coalesce into monsters known as Fiends. Fiends are the predominant, though not only, form of dangerous monster on Spira. Fiends are capable of moving beyond Althena's Boundary or to Filgaia, theoretically, but cannot naturally form there.
Additionally, people who die in Spira without being Sent have a small chance of becoming Unsent. These are undead, where their pyreflies assume a form in the shape of their deceased body. Individuals not from Spira can become Unsent. While Unsent can travel beyond Spira, they are at heightened risk of the the ire of the Guardians. However, they are unaffected by Malevolence or Malice. Being an Unsent, narratively, should be understood to be a liminal state that ultimately ends in a final death.
Spirans who die outside of Spira do not release pyreflies, cannot be Sent, and cannot become Unsent.
The Farplane is an afterlife, where the pyreflies gather and bring the memories and souls of the dead. It is a place of great power and great sorcery, which sits upon an enormously powerful Earthpulse. It is located near Guadosalam, and the Guado have traditionally controlled physical access to it. It is known that the dead in Spira may be visited by venturing to the Farplane.
The massive beast known as Sin emits an effect known as Sin's Toxin. This can cause confusion, hallucination, and sometimes amnesia by those who spend extended time close to Sin. This is not believed to connect to the pyreflies in any way, though pyreflies often surround Sin, as the creature generates large numbers of Fiends known as Sinspawn.