>>283454554
It's an era of peace because she didn't come back for an era.
Typically in fantasy fiction, demons are eternal and unperishable (usually created in prehistory before the world or when the Gods were young etc.) and are elemental evil. They can be banished, not "killed', a term that is meaningless to them in the end. Banished back to the Outer Darkness or Hell or whatever, where they can slowly save up souls or energy or something to try to return and torment your ancestors. That's how it usually goes.
Has Free Ren ever actually addressed the idea that demons can be permanently destroyed, or do they just sort of get kicked back to the underworld to seethe?