>>3844540
In FFX, fiends are sealed in stone just like the fayth. There was the High Summoner Gandolf that sealed the cactaurs in the thunder plains. Of course the spirits of the dead turn into fiends.
But if you play the international/HD version, it's revealed there is a world of fiends "inbetween" the material world and far plain.
There's a series of fiends you have to capture and level up to unlock the Fiend Cup in Shinra's Creature Creator.
The gist of the fiend tales is that 1000 years ago fiends sealed off their world. This leads up to "Ultima Weapon", who is a "traitor" to the Fiends. It's a little confusing. Because the fiends you catch calm to be from another world and that they sealed the path the the fiend world.
However, once you get Ultima Weapon, it says he sealed the path himself to prevent a fiend invasion. But if the path was sealed, how did these otherworld fiends get through?