Shas'O Kais was a T'au Fire Caste Commander and student of Commander Puretide who led the T'au military forces that fought on the world of Kronus during the Dark Crusade. He was dispatched to quell the troubles on the T'au world of Kronus, which had been attacked by the forces of the Imperium of Man, Chaos, the Orks and the Eldar after the awakening of the Necrons who had long slumbered beneath the world's surface.
Kais commanded a detachment of the Fire Caste and the T'au's primary xenos allies, the Vespid and Kroot. He was also dispatched with the Ethereal Aun'El Shi'Or'es to secure and hold the capital city on Kronus and keep the world within the T'au Empire's fold.
Kais was a strong yet merciful commander, often offering his enemies multiple chances to surrender, with their refusal to serve the Greater Good resulting in annihilation. Ultimately, Kais was unsuccessful in retaking the planet for the T'au Empire and was driven off-world by the triumphant Imperial forces of the Blood Ravens Space Marines Chapter, commanded by Captain Davian Thule.
Shas'O Kais was the military commander of the T'au forces deployed on Kronus during the infamous Dark Crusade campaign on that world. He, under the leadership of the T'au Ethereal Aun'El Shi'Or'es, sought to reclaim the planet for the T'au Empire. He is the only faction leader (with the exception of the Ork Warboss Gorgutz), who clearly survives if the T'au are defeated. Should his T'au Strike Force be victorious (the non-canonical ending to the Dawn of War - Dark Crusade PC Game), the T'au colony on Kronus is slowly revitalised as more T'au begin to move in from other Septs.
Humans that fought against the T'au for the Imperium are placed in single gender re-education camps and are treated like second-class citizens of the T'au Empire. Apparently, the T'au are also enacting some sort of sterilization policy to prevent human reproduction -- as the human population starts to dwindle very rapidly as the Kronus colony's T'au population begins to thrive.
Shas'O Kais is deeply spiritual and devoted to the T'au Empire's philosophy of the Greater Good. Unlike many other military leaders in the galaxy, he actually cares about and looks out for the welfare of his warriors. Yet at the same time, he will always offer his enemies, even the Orks and the Forces of Chaos, a chance to surrender.
Shas'O Kais wears an experimental T'au XV22 Stealth Battlesuit, a similar type as that worn by Commander Shadowsun. Through his progression through the campaign on Kronus, Kais can upgrade with a Jetpack, Advanced Sensor Array and Stealth Field Generator. Initially armed with only a Burst Cannon, Kais has access to a plethora of ranged weapons upgrades including Missile Pods, a Fusion Blaster and a Plasma Rifle. He can also be accompanied by various robotic T'au Shield Drones and Gun Drones.
Though there is no evidence to directly support or deny this fact, it is worth noting that Shas'O Kais shares the same name and homeworld as the T'au protagonist from the PC game Warhammer 40,000: Fire Warrior, and may indeed be the same Kais. In the Fire Warrior novel, La'Kais is also apparently driven mad by the events of that story. But in the game, Kais is seen to be mentally stable once more. Also, since Kais uses the XV22 Battlesuit, he could have been promoted to the rank of Shas'o in the Fire Caste after his mental recovery and continued to fight despite the loss of limbs during the events of Fire Warrior. However, it is also worth noting that "Kais" is the T'au word for "skillful", and it is itself a common name in the Fire Caste, borne by Commander Farsight for instance.