Have any Space Marines join the Tau?

Not likely unfortunately. Space marines are born and trained only for war and battle much like orks which is why they have no lasting place in the tau empire and the greater good.

Who can ally with Tau?

So I was reading through the rule book and I saw that the Tau have two Allies of Convenience: Necrons and Eldar.

Are the Tau allied with the Imperium?

Several intelligent alien races (the Kroot, Vespid, Nicassar, Demiurg and even some Humans who have turned their backs on the Imperium, known as Gue’vesa in the T’au Lexicon) have allied themselves with the T’au. The T’au Empire borders the Imperium of Man, and lies within the reach of the Astronomican.

Can Eldar join Tau?

Tau can no longer take Eldar or Space Marines as Battle Brother allies. This for example means you can no longer cast Guide or Prescience on Tau units. Independent characters can no longer join monstrous creatures.

What do Nicassar look like?

Physically, Nicassars have ivory hair-quills and stubby, clawed limbs. They possess no eyes, instead sensing their surroundings with psychic abilities generated by glands in their avian skulls. When communicating with Tau, a Nicassar always floats in a special healsphere.

Where did the Tau ethereal come from?

They first appeared during a siege between the Tau of the plains (later known as the Fire Caste) and the Tau of the city of Fio’taun(later known as the Earth Caste). The Ethereals brought the two sides together to negotiate and convinced them to follow the Greater Good.

What do orks call Tyranids?

Subject: What do orks call the other races? They sometimes call the Tyranids “Gribblies.”

What do Tau call their vehicles?

I find it weird that Tau Vehicles and Battlesuits are often named after aquatic animals (Barracuda, Devilfish, Manta) or other aquatic phenomena (e.g. Riptide).