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This is an interesting and yet hypothetical scenario. But how would diplomatic relations between the Stormcast Eternals and the Tau Empire pan out? How would the chosen of Sigmar react to the philosophy of the Greater Good and how would the Tau react to the revelation that there is a literal God King in existence? How would diplomacy go and would the Tau bring in their forces to try to convert the Stormcast?

Wow, that's a hypothetical that crosses the boundaries of realities right there. Interesting one though, since its not a who beats who in a fight one.

Well, how would the Tau from W40k end up in the Age of Sigmar in the first place?
Experimenting with wormhole technology and accidentally an inter-dimensional hole without ending up in the Warp seems feasible with their level of technology. Maybe using some recovered Necrontyr technology long forgotten by the Necrons.
Or would the Stormcast accidentally end up in 40k due to something going magically wrong with a realm gate? If so good thing they ended up on a Tau planet and not an Imperial one. You worship a hammer wielding God king instead of our glorious God Emperor, HERESY!
Both are kinda interesting to worldbuild on, like those wierd history novels.

As far as I know the Tau do know what Chaos is in a basic sense since they fought them several times. I think they would see the Sigmarites as a more tolerant version of the Imperium of man.
They would get along. The Stormcast would see them as another useful ally against the forces of Chaos and unless the Stormcast would try to smite the Tau. The Tau would simply absorb them in their empire as another auxiliary force. After all have no problem with different religions being worshiped by their acquired citizens as long as it not interfere with the Tau empire at large.

Stormcast aren't remotely xenophobic, so they'd treat the T'au on an equal footing if they approached them peacefully. However if they tried enforcing their greater good, then they would quickly find a hammer headed in their direction.

Given that the only reason for such a meeting would be as the story hook for a crossover battle?
Probably violently.

I could see Farsight pursuing Drago into whatever portal he gets to the Mortal Realms. At first the Stormcast would see a silver armored warrior and be like 'oh, he's a good fella'. Then they'd see him snort warpdust and be like, 'nope', and side with Farsight as his cadre of battlesuits closes in for the kill.