The Deathly Hallowed

Question: Most mythologies have stories of underworld rulers being basically invincible in their realms. In light of this, do you think it would be acceptable to design a Death boon that gives a God bonuses while he is inside an underworld he rules?

Hmm. Not quite that way, but maybe!

Death gods definitely have a leg up when they're on their home turf. Almost no story of anyone, mortal or Titan or god, marching into a death god's underworld with mayhem on their mind ends without said death god owning them so hard they don't remember their names anymore. Mighty Ishtar is laid low when she dares challenge Ereshkigal in Irkallu; Pirithous is imprisoned so firmly for daring to try to invade Hades to steal Persephone that even Heracles can't free him; Baal gets devoured by Mot when he tries to march into Nepesh to cow him; when Hel won't release Baldur, the other Aesir don't even try to go in there. Even those who do successfully bother death gods in their lairs almost always have to survive the encounter by fleeing, not fighting, like Quetzalcoatl desperately racing out of Mictlan or Izanagi making frantic haste out of Yomi. There are your occasional exceptions - Anat defeating Mot while he was still recovering from fighting Baal, or the Hero Twins of the Maya winning out over the lords of death through disguise and shenanigans - but for the most part, it's almost always a very, very bad idea to attack, inconvenience or even annoy a death god while he's in his realm. Death deities have the ultimate power over the realm of death itself, and their iron control over their underworlds is a direct representation of that.

So yes, death gods should definitely have more power in their underworld than out of it. But I'm not sure a boon is actually the best way to go about that. For our games, we assume that most of that power comes from the underworld itself.

In Scion's mechanical framework, an Underworld that belongs to a death god, the way Yomi belongs to Izanami or the House on the Left belongs to Huitzilopochtli, is best modeled as a special kind of Sanctum. It's a Birthright that belongs to that god, inextricably tied to him and sensitive to his needs; it just happens to be an Underworld instead of the Overworld pockets that most non-death gods have as Sanctums. Underworlds vary in their power levels and tie to the gods that own them - for example, Hades is probably a three-star super Birthright, while Teich nDuinn, being less well-known and pivotal in myth, is probably only (only) one.

The general awesomeness of a death goddess while within her Underworld is most likely tied to this Birthright, and thus the underworld itself, directly. That Birthright gives her any one of a slew of bonuses - she has fifty extra soak here, or she can teleport anywhere within its borders for a Legend, or her Strike Dead boon can affect anyone of any Legend, or her Perception is quadrupled the better to keep watch over the place, or every time she creates a shambling zombie she actually creates three. Whatever is most appropriate for that individual death goddess and deathrealm, just like any other Birthright's special powers, should be part of that underworld's innate powers that are conferred upon her as its ruler. While she stands within its borders, she is a force to be reckoned with.

A boon is more flexible, but in this case I think that's actually more of a hindrance than a help. Death gods are the badass masters of their realms, but that refers to their realms, not the realms of others, so a Death boon that just made death gods more badass while in an underworld wouldn't necessarily make sense. If Osiris tried to invade Hel, he should not be on an even footing with her, because this is her underworld and invaders risk attacking her on her own hallowed ground at their peril, mummified or not. A boon that only applied to your own underworld, on the other hand, is curiously limited; it would have to be high-level because only gods can own Underworlds anyway, but it only applies when you're at home, making it functionally useless for Scions most of the time (alas, they probably don't get to do the majority of their divine missions from the cushy comfort of their thrones). And what about those death gods who don't own Underworlds at all, like the Morrigan or Anubis? They'd end up with a boon that was literally, totally and inescapably useless to them forever. There are always boons that are not as useful for some gods as others, but a boon that you literally cannot ever use, no matter how awesome you are or how many resources you blow, would not be great design.

However, if you do really want to add a boon to Death that concerns an affinity for underworlds in general, we could see that as a possible addition. It would probably work better as a Demigod-level power, something budding young Scions with Death can pick up when they start to have to go to underworlds, that provides modest bonuses but doesn't allow death gods to override the true masters of the domain they're visiting. The most major point for that is making sure it doesn't step on Spirit Lamp's toes too much; its powers should focus on adapting and drawing from the underworld itself, rather than avoiding or escaping it the way Psychopomp does.