Dimetor

Question: How would you use Zagreus in Scion? Is he some mysterious child of Zeus, one of the few children of Hades, or just another name for Dionysus?

Oh, man, hide the women and children. The Orphic mystery cults are at the door.

Zagreus, like a lot of mystery cult figures, is someone that every game's going to have to make a specialty call on, I think. The various mystery cults positively delight in deviating from the general canon of Greek myth and establishing alternative geneaologies, powers, and stories for their gods; it's a bit like reading alternate-universe fanfiction, where you know all the characters but they're all doing and saying bizarre things that would make no sense in their normal forms. That's not to say that these cult stories aren't legitimate mythology, because they are; but they're very specialized fringe mythology, and some of them were most likely philosophical or religious ideas rather than literal stories about gods, and we have very little way of telling them apart. Most Storytellers stick to the more mainstream Greek myths, and as a result probably don't even know who Zagreus is; and if you're comfortable with that, more power to you.

But for those who are interested in using Zagreus, he's a complicated, odd figure. Both Nonnus and Diodorus Siculus claim that he is the first incarnation of Dionysus, and that the Dionysus of the Olympians is a god reborn from the heart of the original when he was killed, thus earning the epithet "Twice-Born" (but, in more mainstream Greek myth, "Twice-Born" can also be taken to refer to Zeus having sewn an infant Dionysus up into his thigh to "give birth" to him a second time); further, they note his parents as Zeus and Persephone. The Orphic Hymns sometimes name him as the son of Zeus and Persephone, and at other times only mention Persephone as his mother with no father apparent; Aeschylus, however, refers to him as the son of Persephone and Hades (who was sometimes referred to as the "chthonic/underworld Zeus"). An even older, more fragmentary mention in a lost Theban epic cycle contains no parentage but suggests him as an ancient Titan and partner of Gaia, and he's furthermore syncretized mercilessly wilth various other figures, the most notable being Osiris of Egypt (and, indeed, Osiris is also strongly linked to Dionysus in his normal form as well thanks to syncretization) and Sabazios of Phyrgia.

In short, he's a complete mess.

For most games, it's probably easiest to accept the Orphic assertion that Zagreus was the original Dionysus, and then decide for yourself whether he's permanently dead or truly reborn as the current Dionysus. As far as the question of parentage goes, we're most fond of the idea that Zagreus would be a son of Hades and Persephone; not only is it reasonable in light of the Homerian epithet that links Hades to being called Zeus and the allusions in Aeschylus, but it makes more in-universe sense, as Zeus gave Persephone away to his brother to be his wife, and Persephone is Zeus' daughter (and while he gets very randy with his sisters and other ladies, I can think of no other myth where he goes after his daughters, which might be a level of incest even the ancient Greeks weren't okay with in their gods). If you want to get really awesomely layered with it, you could consider Zagreus the son of Hades and Persephone, and then as new Dionysus the son of Zeus and Semele when he was reborn, making him the legitimate son of three of the major Greek gods in one shot. That theory's one of my favorites, partially because it's a really fun relationship dynamic and especially because it gives you a possible reason why Dionysus' mysterious ability to convince Hades to let him go retrieve his dead mother worked out with no apparent trouble or fallout.

Of course, if all this is too fiddly or uninteresting and it doesn't matter for your games, you could always declare that Zagreus is just an epithet Dionysus has used in the past when he's prankin' people, or even that the Orphic myths about him were entirely fabricated by their members, either on their own or with encouragement from Dionysus himself. Humans do occasionally just make things up, and the mystery cults are definitely a place where that's not an unlikely theory. There's also always the possibility that Zagreus is just a Greek import of the Phrygian Sabazios, and that mortals have syncretized two gods who are actually quite separate by trying to import the non-Greek divinity into their religion.

Zagreus doesn't need to be a major element in anyone's game if they don't have a vested interest in him; aside from his birth, death and rebirth story, he really doesn't appear to do much other than give Zeus an excuse to tell his other kids they can't have nice things. It's easiest to simply leave him out - but if you do include him, he has quite a few meaty possibilities for awesome interactions and history between the gods.

(And this isn't even getting into the Eleusinian mysteries, folks, in which Dionysus has yet another name - Iacchus - and yet another set of parents - Zeus and Demeter, or the Dionysian mysteries themselves, which are basically dedicated entirely to doing weird stuff with Dionysus. This is a guy who really can't stand to see any mystery religion that doesn't feature him prominently and confusingly.)