The Sapphire Guard was the branch of House Illistim responsible for protecting the city-state of Ta'Illistim and the outlying provinces. First established within the city by the Argent Mirror Segeir after the Undead War, where the Illistim had failed to repel the hordes of Despana resulting in damages inside the city, the Sapphire Guard was a mostly "non-magical" military hierarchy which was supported with magical units as well as other branches of the Illistim state. The military during the reign of Lahrair also spun-off a separate legion which evolved into the Sabrar.
While mostly "purely physical" in nature, the Sapphire Guard was equipped by the Artificers, and so had highly magical equipment. The Lord Commander Murstyr Javilerre Illistim, for example, presently wears essentially nothing but veil iron armor which is virtually indestructible. Their hierarchy was loosely modeled on the Crimson Legion of House Vaalor, as the Vaalor were famous for it, so there was no sense in messing with a system that manifestly works. Though there are critics who would disagree, the Undead War being the case in point.
Service in the Sapphire Guard was purely voluntary, thus somewhat more forgiving than the Elemancers, though once inducted as a guardsman there was a hundred year minimum of military service or else they would be severely punished. The intellectual nature of Ta'Illistim was such that there were quite a few master tacticians within the Sappire Guard, and they had magical units posted along the walls for support and firepower in battle.
The Sapphire Guard was the primary military force of the state, but when needed it tapped the resources of other battle-oriented branches. These included the Arch Magi, who were usually the royal bodyguards and so purely focused on combat magic, as well as the Elemancers who were typically the ones performing drastic magic such as summoning legions of elementals. Some Guard members actively sought out ways to develop new tools to improve their effectiveness, even consulting with Withycombe gnomes, wishing to incorporate machines within weapons and defensive measures.
"Illistim Culture - Part II: Branches of the State" - Haethera Illuvassas Illistim (5109 Modern Era)