Play It Again, Sam

Question: Is reincarnation a thing in your games, or do all dead souls end up in an Underworld permanently?

It is indeed a thing, but like all things in Scion, it depends on the myths of the culture in question!

Reincarnation was an integral part of the afterlife beliefs of many ancient religions; Hinduism and Buddhism (up to the present day!) are the most obvious examples, but it applies to several others, including the Gaulish celts (who believed that the greatest heroes could be reincarnated to fight their battles anew), the Norse (who believed that particularly deserving dead people could be reborn as new creatures like the valkyries), and even the Aztecs (who believed that certain classes of people might be briefly reborn in new forms, like warriors that returned to the earth as hummingbirds). Of course, the majority of Scion's religions didn't believe in reincarnation and do prefer the model where the dead are locked away in the Underworld forever; this has to do with ancient fears of the dead interfering with the lives of the living and is also a perfectly valid way of approaching the afterlife, but the two can coexist with a little in-game creativity.

Like most things that surround death and the souls consigned to it, the best place to handle questions of reincarnation and eternity is almost always in the Underworld or the god who rules it. The Scion books (with the exception of a side mention in the Shen underworld of Di Yu) tend to shoehorn everyone into the underworld-forever model for conformity's sake, but the Underworlds of pantheons who espouse reincarnation are the perfect place to provide mechanics and possibilities for riding the wheel of incarnation again. Di Yu should house the great wheel of reincarnation, accessible only to those who have come through their purifying punishments intact and ready to attempt life again; the souls of Naraka should be brought up before Yama for judgment whenever he deems it appropriate and then sent back to the World in whatever form he considers them worthy of; the warriors of the House on the Left should return to earth when their formidable master commands them to and not before.

The best way to handle this is probably with some sort of Underworld feature that accomplishes reincarnation; the wheel of Di Yu is a great example, since it's a permanent feature of the Underworld that exists specifically to accomplish reincarnation and keep the cycle of lives going. We'd consider it either a massive multi-star relic or a part of the Underworld itself, usable in whatever ways and by whatever people the Storyteller considers best for the tale at hand. For religions that don't have easy features like that to mechanically explain how reincarnation works, you may want to invent some - after all, just because Gaulish myth doesn't actually say there's a tree/well/wheel/river/whatever of reincarnation doesn't mean you can't invent one to explain how the process of getting heroes back into the World actually happens. The Underworld is the implied place where all disposition of dead people occurs, so extending that to include their return to the world only makes sense.

The other option is that, in religions where there is clearly a form of reincarnation but also clearly no mechanism in the Underworld to explain it, it's simply the death god him- or herself that is accomplishing the task. Resurrection of the dead is difficult and not to be taken lightly - in most cases, it would require The Reaper - but it's still part of the job for gods whose afterlife duties include overseeing reincarnation. Some may use their Avatar once in a blue moon to reincarnate the particularly worthy; others, who operate in mythologies where many souls are reincarnated, may do it regularly in a large cattle-call and then rest up for a month before going at it again. Just as the death gods who manage Underworlds where no one is allowed to escape to trouble the living have a serious responsibility to ensure that that never happens, so the death gods of the reincarnation pantheons have an equally serious duty to ensure that those who are meant to walk the earth again do so. Being a god of death, especially one who administers the Underworld, is one of the heavier responsibilities in any pantheon, after all.

So yes, reincarnation can and should happen in Scion, where it's appropriate. It will seldom be a get-out-of-Di Yu-free card for Scions, since few religions believe in instant reincarnation and instead require the dead to undergo a waiting period or trials of purification and judgment first, but it's still out there for all your Storytelling plot needs. Scions angling to become gods of death themselves might be particularly interested in it, since they'll need to decide if they're going to be reincarnators or jailers, and what each different kind of Underworld control might mean for them as deities.