Militarum Tempestus

"Death is not failure, for even death can bring glory. Fear is not failure, for fear can be conquered. They only known failure is to ignore orders, for even a slight hesitation in following them brings an ignoble end"

- Liber Progenium, Volume 1

The Ordo Tempestus is amongst the most rigidly codified of all Imperial organisations, for its men form the backbone of the Astra Militarum. Though the ordo is technically a sub-faction governed by the Adeptus Administratum, it enjoys a far greater amount of autonomy than the regiments that often fight alongside it. The ordo's ranks are primarily comprised of Commissars and Tempestus Scions, though they have often included specialist factions mysteriously absent from Imperial records. In every theatre of war across the galaxy, the ordo's men work alongside the incalculable might of the Astra Militarum, their elite training complementing the sheer manpower of the Imperial Guard.

If the ordo provides the rigid skeleton of discipline that holds the Astra Militarum together, it is the Commissars who are the minds of the operation. The Officio Prefectus governs and controls the regiments of Tempestus Scions and Imperial Guardsmen alike, ensuring that their military force is put to the right use in the Emperor's name. All Commissars are trusted to improvise new orders on the battlefield, a rare privilege in the rigidly controlled structure of the Imperial war machine. But it is only the most senior of their number, known as Lord Commissars, who are truly independent. They are warriors of great personal charisma, and they will often inspire the men by leading from the front rather than from behind the barrel of a bolt pistol.

The Tempestus Scions do not form the main body of the Astra Militarum, for that duty falls to the regular Imperial Guardsmen. Instead they can be likened to a knife, a thrusting point of lethal force that is applied with shocking speed into the foes's weakest point. Many a grinding war of attrition or extended campaign has been brought to a dramatic close by a strike force of Tempestus Scions. More often than not their insertion, mission completion and extraction parameters are all accomplished on the same day.

Source

 * Codex: Militarum Tempestus (7th Edition)