The original lore had the Ethereals appearing in one single night, following a series of strange lights being seen in the air at night. They took control of Tau society instantly as Tau instinctively followed them. There was also an autopsy where they discovered the difference between ethereals and other Tau was the presence of a gland in the brain that came from another animal from a planet a long way away, which couldn't possibly have naturally evolved.
There was other stuff as well - it was never directly spelt out, but it seemed to be clear that the Tau were artificially supported in some way, and the Eldar fit better than anyone else.
(Remember,. their entire society in the original lore appeared basically overnight. They went from medieval to space flight faster than thought possible).
That isn't
quite true. The original Codex said there was a
legend among the Tau where those things happened like that, not that it was inherently true and all that actually happened as the story says. The "Greater Good" and running their Empire relies on the Ethereals being viewed as supreme and utterly neccesary by the Tau, so if their unquestionable, infallible leaders had a typical backstory in Tau culture didn't elevate them about the others, they'd seem a bit too "normal".
Something else to consider, it's unlikely they'd say the Ethereals didn't fix things (E.g. they arrived but it took centuries to stop all the wars) in their legends because then they don't look perfect.
As for the information in Xenology, that was from an in-universe source that gets information wrong, speculates heavily and ultimately is not a reliable narrator.
They're In-universe speculation and centuries old legends from Tau Culture, from what i've seen there is no actual evidence for it that's reliable. They suggest the possibility, but that doesn't mean it's actually true.
With "They went from medieval to space flight faster than thought possible" though, isn't that what happened with us? From the discovery of blackpowder (the Tau were said to have Cannons when they Ethereals arrived) to spaceflight in around 1000 years, which from what i remember isn't that much different a time-frame than the Tau achieved it in (Up until a few hundred years before the current W40K time they were still using Fossil Fuels, even). The Ethereals allowed the Tau to work together, so there was no period with lots of wars and such after that, supposedly.
So now we have come full circle. That's such a blatant copy of vampire hunter Valla from Heroes of the storm I'm surprised they aren't talking copyright. Blizzard copied from GW, and now GW copies from Blizzard.
Am i missing something? This is the character you mean, right?
The only similarities i can see are single-handed crossbows (which aren't unique to either of them), the hat and high-heels. Both of them have a typical daemon-hunter theme, which that hat is associated with. I don't see how Greyfax's slightly steampunk-gothic ornate witch/daemon hunter armour matches that character.
