This is a Fan-made piece of work.
During the T'Ros campaign an Ethereal, called Aun'Vre, was killed by the hand of an Imperial Eversor assassin. This sparked rage and fury in the hearts of all the Fire Caste fighting there, especially the Commander, Shas'O R'Myr. Many of the Fire Warriors never fully recovered from that period of rage and vengeance.
One of such Fire Warriors, serving in O'R'Myr's own Cadre, was a young and promising Crisis Battlesuit team leader. His hatred against the Imperium never fully went away, neither did the hatred of his bonded team mates, Ui'Korst'la and Ui'Shas'Myr. When T'Ros was secured, the Cadre was transfered elsewhere. After several years the team was assigned to another Cadre and the warrior, already a Shas'Vre team leader, fought again against the Imperium of Man.
This warrior became kind of a specialist in defensive tactics, incorporating both doctrines of war, Kauyon and Mont'Ka. When Shas'El Va'Suam, leading his cadre during the failed defense of Mal'caor Outpost, was killed in combat by a Krak Missile, fired at her by an Ultramarine Space Marine, the warrior was promoted to a Shas'El and given the command of the Cadre.
The newly promoted Shas'El then contacted an old acquintance and battlemate, Kroot Shaper Garrok, whom he had met at T'Ros, and asked him to join his Cadre. Garrok happily agreed and took with him a whole kindred of his Kroots. These Kroots proved invaluable later, during Marna V campaign.
It was during the bloody Marna V campaign that the hatred against the Imperium fully woke up again. After having fought the Ultramarines for several days, the Shas'El had seen so many Tau slaughtered, that he and his bonded team mates decided to give no mercy to any Imperial warrior. Normally, such behaviour would be seen as improper for a Tau, however, the situation on Marna V was deteriorating quickly with newly arriving Imperial reinforcements and the Ethereals decided to try to use the hatred to actually aid the struggle for the Greater Good.
During the defense of Saint Dizier repair depot on Marna V the Shas'El showed such tactical and leadership skills and stern determination in organizing the defense and evacuation, that he earned another promotion and a new name. From then on, he was to be known as Shas'O Sa'Cea Lar'Shi'el Kir'qath Ny'oni, or Commander Watchdog, as the Imperials quickly learnt to call him.
The Ethereals were so impressed by O'Lar'Shi'el's battle performance that they agreed to the proposal of the Earth Caste, who were grateful for the saving of many of their personnel at Saint Dizier, to allow the Commander to use the same type of experimental Battlesuit as O'R'Myr. The Earth Caste even insisted on O'Lar'Shi'el's bonded bodyguard team to be equipped with the rare XV-81 and XV-84 Battlesuit variants.Hunter Cadre Lar'Shi'el soon earned a reputation on both sides of the Marna V frontlines. To make the best use of the reputation, the Ethereals agreed on the Commander's proposal to change the paint scheme of the Cadre from the usual camouflage pattern to one that could easily be recognized and feared by the warriors of the Imperium of Man. It did not take long for the Imperial Guardsmen to shiver in fear any time their attacks were faced with Fire Warriors, Kroots, Battlesuits and Hammerhead tanks, bearing the easily recognizable paint scheme of silver and Sa'Cea Sept's orange colours. None of these attacks managed to break through Tau lines and all were halted in a bloodbath...