Ethereal
The Ethereals (or Aun in Tau) are the leaders of Tau society and the existential space popes of WH40K (Ministorum Priest not counting). They are infamous for their supposed 'mind control' powers and their ability to inspire Cadres to fight with the religious tenacity of the Imperial Guard whenever they personally take to the field.
When we call Tau the blue space commies, Ethereals are the ones who are "more equal than others", as that old satire goes. Every single Tau loves these guys, seeing them as embodying all of the Tau species good traits and avoiding all the bad ones. Normal Tau are, despite being (almost) immune to Khorne, no less prone to being a bunch of spiteful violent fucks when left to their own devices for a few decades. Ethereals never have this problem, always in control of themselves and moving with a grace that just says "I'm in control of the rest of this shit too. Just chill out, space dad is here."
Ethereal Guards
The Ethereal Guard, also known as Tau Honour Guards are Tau warriors who serve as the bodyguards and protectors of Ethereals, the undisputed leaders of the Tau Empire. These warriors are dressed in ritual garb but wear Recon Armour and wield Honour Blades. Suffice to say, they seem mostly style over substance and are in no way in the same league of the Adeptus Custodes.
History
Their arrival started the unification process between the various Tau tribes and coincided with the implementation of the concept of the Greater Good. 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. Their power over the Tau is such that an Ethereal could tell a Tau to die and that Tau would do so quickly and gladly. The Adeptus Mechanicus would give much to learn the secret of the Ethereal's power. There is strong implication from the similarities between an organ possessed uniquely among Tau by the Ethereals, and an organ which Q'Orl use in pheromonal communication that this ability may be due to deliberate engineering in the Ethereals' past.
The Ethereal sometimes lead the fire caste into battle to provide inspirational leadership, so the warriors can achieve more. However if the Ethereal falls in combat, the remaining Tau are struck with grief that causes them to lose battles or they get extremely enraged and charge forward, either way after a Ethereal falls the Fire Warriors lose all discipline and efficiency, which can be detrimental to any conflict if a commander can exploit it.
Though the obedience of the Tau to Ethereals is natural, the other races on the other hand have long been speculated to be some sort of covert mind control. There are a lot of theories from the prenatal indocrination or pheromonal compulsion to the presence of some veiled psychic gift possessed by the Ethereals, but as yet no confirmed proof of any such power has been secured. Those few Ethereals that has been captured and subjected to interrogation have proven less than forthcoming. Additionally, given that Farsight and his enclave managed to break away from Ethereal control; it is highly probable that total obedience to the Ethereals are indeed cultivated through artificial methods.
On their foreheads resides a small organ, smaller than the ones the other castes have. Imperial studies on the organ show no biological function, but it's suspected that the ethereal studied is a dud, or that the way the Ethereal control the Tau is more esoteric in nature.
As the leaders of the Tau, the Ethereals are enigmatic and studious, yet unfaltering in their authority and their drive to further the Greater Good. They are mystics and philosophers, possessed of knowledge and wisdom not shared by their more practical subjects. As such they had some part to play in the Tau rapid development, and to this day guide their species in its dynamic expansion, or the castes revert to the savagery that once threatened to destroy them.
Killing an Ethereal
This being 40K, there is no small amount of writing on what happens when an Ethereal gets blown the fuck up on the field of battle. Without fail this sends the rest of the Tau soldiers into a brief panic attack and state of intense grief. The enemy forces at this point have a great opportunity to capitalize; if they move quickly enough they can butcher and route the rest of the Tau in short order. If they aren't quick enough, however, the Tau will replace "panic attack" with "just fucking attack" and set all their guns to full auto, and unload bullet after bullet into the enemy who closed the distance right into maximum rape range thinking they had an easy victory.
The crunch has gone through a few different phases of representing this. In the past, a dead Ethereal would mean all remaining Tau forces gain Preferred Enemy, but also have to take a leadership test and thus risk running off the board. This wasn't really fun, since the Ethereals were pretty damn shit in those days anyway, and your options were either find a way to get the Ethereal killed right at the start (when you're more able to regroup a fleeing squad) or just take a Shas'O instead you idiot. Starting in 6th edition they changed things around to be less fluffy but also less shit: now, the Ethereal himself gives buffs to his army, but give the Tau's opponent scores a bonus victory point if they manage to kill the Ethereal.