i think a good fluff story would have to be centered around a great curse.
back during the horus heresy a might battleship fleet was lost to the warp. With in the confines of the mighty vessels of the imperium where the Kheri Desert Fighters 121 Regiment. These brave men were forced into the Imperial Army to fight against the treacherous Horus and his followers.
The stalwart navigators of Kheri were doomed as they began to find themselves lost within the warp. their powers falling them as the ghellar fields began to crumble allowing a host of daemons entrance into the fleet.
The soldiers fought back with tooth and nail not knowing how the mighty shield had fallen. It was only too late when a great lord of decay forced its way into the bridge that the soldiers saw the traitor among them. A foul sorceror had sabatoged the mighty vessels and even now was begining a great and terrible curse that would doom the entire crew and the soldiers.
Before the curse was complete the regiments Senior Officer had fought his way to the sorceror and before the final incantation could be brought forth he killed the wicked one. Despite his bravery the curse was still in effect and their flesh began to rot away before their very eyes. Despite these terrifying happensing all around them they managed to keep their humanity thanks to the acts of the officer.
The Kheri regiment is said to appear from the ground on battlefields as they would back on the loose sands of their homeworld erupting from the ground grasping at their opponents and destroying them with a cold mechanical will that only survives to serve the emperor!
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