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.
Back then, killing a Tau army's Ethereal would cause the Fire Warriors to break down in tears and be vulnerable to being unceremoniously torn to ribbons a few moments later. Recent updates in both fluff and crunch, however, made Etherals less of a liability to a Tau army's morale. There have been cases where the death of an Ethereal would actually cause their attached Hunter Cadre to be pissed off to such a degree that they'd fight like angry gun-toting weebs that had their waifus insulted. This is somewhat reflected in crunch aswell, as killing a player's Ethereal grants you extra victory points and cancels the space pope's ability bonuses. You can be assured your opponent will attempt to gun all of your forces down with extreme prejudice to make up for this.
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.