Question: Do the Nemetondevos have an Overworld?
Gaulish cosmology party!
Or rather, lack of Gaulish cosmology party. As you note, we don't actually know... well, anything much at all about the cosmology of the ancient Gaulish religion. Most of our notions of what the ancient continental Celtic conception of the universe might have been are reconstructed and piecemeal; we put things together based on fragments of ancient art, possible related concepts in the iconography and symbolism of various Gaulish deities, and even comparison with other nearby cultures' cosmologies. Some scholars attempt to reconstruct a three-part world from the Gundestrap Cauldron, one of the more famous pieces of Gaulish art:
The theory is that the scenes - which include a clearly identifiable image of Cernunnos, as well as other figures that are theorized to be Taranis and Belenos - show symbolic depictions of three worlds, including a world of humanity, a world of the gods and an "otherworld" beyond it that might be the home of monsters or other strange creatures. The theory is heavily based on drawing parallels between the worlds of Norse cosmology (Midgard, Asgard and Jotunheim) and the worlds of Irish cosmology (Earth, Tir na nOg and Faerie), and while it does have some interesting possible parallels (like the guess that maybe the giant bovine figure on the base of the cauldron is the same as the Norse Audumbla, or the idea that the scenes are meant to depict battles from the Tain bo Cuailgne), in the end it's just a big old pile of guesses. We have no idea if any of them are correct, despite the fact that various scholars with wildly different theories are willing to duel to the metaphorical death over their own personal favorites.
The Gauls were largely pre-literate and what few things we have preserved from their religion were recorded by Roman invaders, which means that they're all highly polluted by syncretism and interpretatio romana, and most of the details that didn't match up to Roman religion were summarily ignored. Julius Caesar, our best written source for Gaulish religion (and isn't that a depressing thought?), describes a few of their gods, funeral rites and sacrificial rituals, but he doesn't bother to explain how they thought the universe was put together. Which in itself sparks more scholarly theories that maybe he didn't describe it because it was very similar to Roman cosmology and he assumed his audience back home would automatically assume that... but really, we have no clue.
So the upshot of all this is that Gaulish cosmology is a giant question mark, and has been for centuries. And unless we discover either an ancient writer's hidden works that describe the religion in detail, or some previously unknown artwork or artifacts that directly depict it, that's the way it's going to stay. A moment of silence for the things we have forgotten and may never be able to remember or reconstruct.
This is one of the major reasons that the Nemetondevos are hard to work with in Scion. We don't know very much about them, and even less about the universe their people believed they inhabited; and that means that we don't have much to go on when we try to figure out what associations their gods have, where their Scions might go, who their enemies might be or what kinds of stories and mythic archetypes best describe them. They're essentially a lost pantheon, which is why the official supplement that details them basically approaches them from a perspective that says that the pantheon was almost eliminated, has been out of play in Scion's universe for a long time, and has as its greatest enemy Fate itself. If anything can erase a pantheon, a group of beings literally made up of pure legend and myth, it's Fate. Fate alone can erase the stories of even the most incredible mythic exploits, so that's what the supplement suggests happened to these gods.
We pretty much agree with you, first questioner; Dubnolissos is a pretty good shot at a posited Gaulish Underworld, but at the end of the day it's almost totally fabricated. The supplement leans heavily on the idea that because the Nemetondevos attempted to oppose Fate itself in the person of the Matrones (whose trinity of Fate-weavers is another repeat of the European idea of the three women who represent destiny, just like the Moirai, Norns or Sudice), Fate lashed back at them by wiping their mortal worshipers off the map via the conquest of Gaul by Rome, and wiping the gods themselves from the history of the world. As a result, Dubnolissos is written as a place where all the Gaulish gods reside with their people - forgotten gods together with their forgotten worshipers. It's designed as a larger-than-life version of what the Gaulish lands would have looked like in the time of the Nemetondevos' heyday, complete with wildlife and villages of mortal souls. In keeping with the theme of the Gauls being tied to their people, which allows them to be wiped out when their people are wiped out, the PSP of Deuogdonio also depends on affecting mortal worshipers, and all of the gods' territories in Dubnolissos are based on the idea that in time gone by they actually lived among mortals rather than in an Overworld away from them.
And you know what? That's a pretty goddamn solid and resonant constructed cosmology. Oh, it doesn't necessarily have much to do with what Gaulish mythology was doing two thousand years ago... but it's still a coherent combination of religious concepts and Scion's mechanics that gives Storytellers a lot to play with. If you're going to write up a pantheon that has almost no history left, this is far from a bad way to go.
So we're cool with Dubnolissos, as far as it goes. We don't actively use the Nemetondevos as parent gods because of their lack of context, which makes it hard to write stories that mesh well with other pantheons that do have a lot of history and symbolism to draw on, so we're not going to leap to use them no matter how much we admire the writing job on them, but that doesn't mean we can't give the writers over at Biblioteque Interdite some mad props.
As for the Overworld, second questioner, we don't have any more historical or mythological idea what's going on with that than we do for Dubnolissos... but if you're a fan of the universe that the writers of the Storyteller Screen created for the Nemetondevos, they do have an answer for you. While they never officially published it, the same writers later released a tiny additional supplement that did add an Overworld to the Gaulish cosmology, and explained why it wasn't included in the original pantheon release. In a nutshell, the Titan who was father to the Nemetondeos, Orgos, also pissed off Fate, which responded by actually killing him as he gave birth to the last of the pantheon; his death, as with the deaths of all Titan Avatars, had horrific destabilizing consequences on the universe, and destroyed the pantheon's Axis Mundi (Orgos' throne) and completely severed the Gaulish Overworld of Albion (meaning "white world", according to at least one reconstructed etymology) from the World, making it inaccessible and marooning the Nemetondevos on earth with no way to return home. This is particularly interesting not only because it continues to carry on the theme of the Nemetondevos as a whole being locked in a serious conflict with Fate (which always defeats them - and is perhaps a cautionary tale to other pantheons who think they can overcome Fate's dictates?), but also because Albion is a traditional name for the island of Britain/Scotland, suggesting that the story might be an allegory for the Gaulish pantheon's split away from their "relatives", the other Celtic gods. Irish mythology in particular loves to set Overworlds and other magical locations as faraway islands, so this is an interesting mirror of that concept.
If you'd like to read the supplement, unfortunately we don't have it in the original French; Biblioteque Interdite has since stopped handling Scion's French distribution in light of the impending second edition of the game and its game supplement site is no longer online. However, we did download it and do a quick and dirty translation into English back in the day, which you can read in all its poor-grammar glory here. Please pardon the bad translation - I could probably do a better job if I retackled it now, several years later, but unfortunately I no longer have the original.
We've actually used the Gauls a little bit in our games, despite their near-total dearth of surviving mythology; the idea that they're a largely dead pantheon that serves as an example of the eventual downfall of pantheons that attempt en masse to fight Fate is a resonant one for young Scions, who are in their own ways learning to make their own Legends and fight their own destinies, and the potential continuing cycle of downfall extending to the Theoi, who conquered the Nemetondevos but are guilty of trying to avoid their own Fates as well (hey, there, Zeus, how's that belly full of Metis?) is a fun one to play with in stories. They're mostly gone now - they were already on their last legs, and Ragnarok took out all but one or two of them - but they were a fun experiment while it lasted.
