And here are a few flashbacks from Destroyer of Light.
It is not agreed how the notion of reincarnation arose in ancient Greece, but it seems to date back to the 6th century BCE, with the Pythagoreans and the followers of Orphism. According to myth, Dionysos wandered through many countries before he was recognized as a god, and some say he visited India. So I had the idea to let him pick up the idea of reincarnation there.
“The Lake of Memory” is frequently mentioned on Bacchic gold tablets, but it is almost unheard of outside these tablets. They were put in graves as a kind of instruction for the dead on how to behave in the Underworld. It is important to not drink from the wrong spring, otherwise the dead would lose all their memories (which is the normal thing according to the older Homeric notion of the afterlife).
I think there are two possible interpretations about Dionysos and mysteric religion:
On the one hand, one can think of these "life after death" religions as very ancient, and the Greek civil religion as a sort of "screen" that hides them; from this point of view Zagreus is the "original" Dionysos, and the dude whom puts water into wine is sort of a rationalization of a more "primal" figure.
On the other hand, it is also possible that these views as the Orphic ones are the "new" religion, that the Greek civic religion is the original form, and that these Orphic-like views emerged and became more prominent in time, until they became the late antiquity Mysteries that, ultimately, are way more similar to Christianity than old Greek religion is similar to Christianity (I'll stop there).
What is your opinion, is the mystic and rather dionysiac mysteric cult the older cult or is it a later development?
Good question. In the books I have read it says that Bacchic/Orphic mysteries can be traced back to the late sixth and early fifth centuries. At least by the fifth century, groups who called themselves "Orphics" were representing Dionysos as an eschatological deity.
Sarah Iles Johnston argues in Ritual Texts for the Afterlife that the Orphic myth and cult was created around that time, probably inspired by the older mystery cult at Eleusis, since no trace of the myth can be found before that. She says it is also significant that when authors refer to the cult during the fifth and early forth centuries (i.e., within a hundred years after the cult's apparent debut), they treat it in ways suggesting that, in contrast to the Eleusinian Mysteries, this cult was far from being well entrenched and respected.
But maybe other scholars have different opinions? I'd like to read more about Orphism. It's such an interesting, confusing mess. ^^
Also, I've got the impression that Zagreus and Dionysos was not one and the same from the beginning. There are apparently quite few Orphic texts that refer to Dionysos as Zagreus, except for Nonnos' Dionysiaca, which is a quite late source. Then there are a few earlier fragments that possibly identify him with Dionysos. A fragment from Euripides' lost play Cretan Men mentions "night-ranging Zagreus, performing his feasts of raw flesh" (which is suggestive of Dionysos).
I do not know much about orphic mysteries either, however Herodotus (5th century) says that the 3 most important Egyptian gods were Hephestos, Demeter and Dionysos. I assume that he meant Ptah, Isis and Osiris, also because Osiris was later syncrethised with Dyonisos, so this makes me think that the linkage between Dyonisos and Osiris is quite old. The death of Osiris is similar to that of Zagreus too. But I can't tell if this had an eschatologic meaning since the beginning, which is my curiosity.
Disclosure: I'm somewhat curious about mythology but I don't really know much about it, many of my readings consist of now discredited stuff like Graves or even Frazer.