Tau Ethereals were literally made by Eldar (
or were they) and I feel like there shouldn't be any discussion about it. A lot of
Weebs people (on both sides, people also don't like to be associated with Tau too much if they don't like them) hate the idea because it means that Tau aren't as cool if everything they did was basically handed out to them by much more advanced race, but tough luck. They're living in an Universe which backstory is so mind-bogglingly deep that it goes back before it was even created and they're one of many minor races and yet they get a lot of spotlight (of course I know this is a war game first and foremost so it's all due to SALES and MONEY, but sometimes I like to think that lore is important, since I know so much of it) - they surely must be important in the greater scheme of things to get such attention.
This is actually not Q'orl Queen and I can't see the crystal, so this one may not have it. I haven't read Xenology in long time. Note how Q'orl space is actually visible on the Galactic map,
unlike Tau, which says something about how overhyped they are... unless they serve some higher purpose...
Basically, one of biggest arguments for that the Tau are engineered race is that Q'Orl (hivemind giant ants minor race) has literally identical crystalline organ used for mind control despite being far, far away. They have an historical writings that they mention that some time ago they basically they were fighting Chaos and then some alien humanoids came and helped them and in return they stole the Q'Orl queen in order to "make uncorroded swarm" (as in - race resistant to Chaos). Many belive it was Eldar who then made Ethereals and dropped them on Tau. To anyone saying that they do not benefit in any way from this and it's actually counter-productive, I must remind you that Tau to this day remain tiny fucking fries and they in no way threaten either Imperium or Orks. Hell, Q'orl would be better choice to fuck up Imperium since they're much closer to Terra (and Eye of Terror, getting at Cadia Gate from the back when) and actually have meaningful territory (which is what actually almost happened, if not for that secret force that saved them from it). Eldar also benefit from the simple fact that it's an race that is very goddamn fucking resistant to Chaos corruption (which is the sole reason they don't know much about Warp, they just don't really "feel" it and have no psykers), because they have both test lab and they could possibly plan to implement the Tau as front-line troops due to their near-immunity while Eldar psykers that are also pretty resistant due to sheer willpower would deal with actual "magic" stuff.
Though, in my humble opinion it was actually Necrons - after all, there are the physical similarities of Tau to Necrontyr - especially Ethereal old wrinkly dudes with vaginas on foreheads (containing the mentioned crystal) and being the only Tau with regular feet (which everyone seems to forget all the fucking times, hell, even the official minis have hooves). There are more similarities, long skulls, long faces, pronounced cheekbones (it should be noted that the Earth/Fire/Water/Air castes have much more rounder faces), short lives... and hell, the general look of Tau seems like well-fed not-dying-of-radiation Necrontyr, not to mention the very similar vibes when it comes to clothing and whatever stuff they use, though that can be explained by but GW lack of creativity. The timeline also checks out - the Q'orl queen was stolen about at the time when Imperium discovered Tau as cow cavemen and soon after covered in very convinent Warp Storm (I would want to remind you that Necron weapon against C'Tan was Blackstone Fortresses and those are shooting literal Warp - since C'Tan couldn't exist in Warp they got shrekt) and honestly, the Tau are very similar in their relationship with Warp as Necrons - except just that they didin't do any extensive research into Warp stuff which doesn't mean they won't. I should also mention that it wouldn't be the first time they did something like that - the human Pariah Gene, the one that is responsible for existence of Blanks, was deliberately engineered into humanity by Necrons back during the War in Heaven. There is still question on why would they do it, and while I am honestly confused on the current Necron lore (it's literally my weakest point because I usually tend to gather all the lore from all editions together and Necrons were changed VERY HEAVILY) I actually do support the part that Necrons themselves wouldn't get much from it (maybe unless they hold the mind-control button to turn the Ethereals off and just wait for Tau to conquer the Galaxy so they can come up later on and cackle maniacally as they win everything) themselves, but... then there's the fucking Mephet'ran, better known as The Deciever.
You know, the dude that made a deal with Cegorach and caused the C'Tan to fight among themselves and indirectly caused their sharding in apparently not-as-planned episode and ended up as the laughing stock of the "real" trolling masters like Creed, Tzzentch, Cegorach and Big E (remember that he stole some of his plot powers from Chaos Gods)? The dude that keeps popping up at places in which he shouldn't (convincing the Failbaddon that the 12th Black Crusade is good idea and Blackstone fortresses are cool) and having great fucking time trolling everyone while also setting up completly incomprehensible schemes? The dude that keeps giggling about the "Seed in The East"? The dude that basically only real motivation is to come out on top no matter what and the dude that is one of few beings in existence that is actually cappable of 'Out-As-Planning" everyone? The dude that already had control over very powerful advanced race and fucked up because he didin't have an efficent way to force them to be obedient? Oh, of course Chaos might see it coming which may explain the deamon attack on that Tau leader and their tries to get the insanely resistant Tau on their side (Shas'la Kais, though considering his uncommon sanity for a Tau after he gets a promotion to Shas'o in Dark Crusade, that might actually mean it backfired).
And this is where we get to actual conspiracy and where the great overarching (WH40k is literally overarching plots overarching overarching plots, ugh) plot thickens. This is basically a theory of mine with which I came up and doesn't have that much with Tau so you can skip it, but the feeling of "JUST AS PLANNED" was too heavy to not become a writebunchofasticks just for a while. Basically the idea is that since the War in Heaven The Messenger (latter known as The Deciever) has been doing all his unexplainable stuff just to fuck up with Chaos. He struck a deal with Eldar Laughing God to convince the other Star Gods to start eating each other out, but ultimately was betrayed by him (which later came to (quite literally) fuck Cegorach in ass, which means he proably learned his lesson now), possibly knew that The Silent King is going to betray C'Tan all along, thus was indirectly responsible for Sharding which doesn't sound that bad if you consider that majority of free Shards seem to belong to Mephet'ran or that it's implied (stop appearing fucking everywhere you cuck) that he may have not even been Sharded and just keeps up with the act to seem that he is no longer a threat to anyone, that he is out of the game which allows him to appear everywhere and fuck things up. BFG:A is one of most prominent and recent examples but not only one.
It seems that there have been many tries to reinforce the Anti-Empyrean abilities amongst the Galaxy residents, to mention just the Human Blanks, the whole 12th Black Crusade which removed only thing that could have hurt him from play and allowed the status Quo to continue for a bit longer and of course the sole reason we're having this wall of text, the famous "Seed in The East". The Tau. I am not sure in what way are they actually special - are they supposed to be great warriors that will be heavily immune to Immaterial influence and thus be able to fight Chaos effectively? Will they somehow convince the other factions to get together (hehe, maybe if Emperor comes back but that's unlikely) and bash some Chaos fucks? Maybe there is something else in there, some hidden ability? Maybe the Shas'la/Shas'o Kais wasn't the only one and Chaos corruption will be actually beneficial for Tau? Or maybe they are literally the "Seed" that is waiting for the Tyranid harvest that might somehow cause the Tyranids to become the final solution to Chaos? Whatever happens, the Tau are a ticking bomb that barely anyone seems to be aware of, that has been planted by one of biggest decievers in the Galaxy, and that is... never going to go off because we can't advance the plot, but if somehow someone at GW decides to bring "End Times" to Wh40k, I assure you guys that it will be the biggest "JUST AS PLANNED" fuckfest that has ever happened. It's so big that anyone can still come out on top in the end.
Of course, except Tau because fuck Tau. :VFuck I just spent like four hours writing this but I am very afraid to go to sleep ;-;EDIT:
The third spoiler was not as planned.