My assumption was that when they were in Avernus that over time they were morphed from the energies of hell into Tieflings because all information I've seen on Elturel was that it was a largely human city, though I haven't finished reading the Descent into Avernus book
Fiends are spawned in the hells, the abyss, or outer planes like gehenna. Devils are spawned from souls in the river styx and get promoted upwards and morph into other various types. Demons are created out of evil souls, chaotic energies, and the general malevolence of the infinite layers of the abyss. Tieflings are the offspring of one of the creatures breeding with (usually) a human. They have a fiendish heritage. There are no unwilling devils in hell, you cannot be unwillingly taken. Asmodeus, ruler of the hells and one of the ancients before the gods whos awakening will break the hells and take out one of the anchor points of all creation and create Armageddon and revert the universe back to its primal energies so he can do his thing again - yes that asmodeus, agreed to that with the angels of the upper realms in Mechanus that any souls given to devils were to be willing. Excep for atheists
they go straight to Nessus for him to devour.
Anyway, tieflings are offspring.
