The Cuis clones were six prototype clone stormtroopers, bred from the genome of Sa Cuis, a Dark Jedi who served as one of Emperor Palpatine’s personal assassins. Through their progenitor, they gained a strong connection to the Force, thus making them a unique batch compared to the rest of the soldiers in the Stormtrooper Corps, all of whom were cloned from the templates of ordinary Humans at the time. Their creation and training were closely monitored by the Emperor and his apprentice, the Sith Lord Darth Vader, as part of an experiment to create a battalion of obedient clone soldiers trained in the ways of the dark side of the Force. In addition to his connection to the Force, Cuis was chosen as a template by Vader because of his sense of loyalty—a trait that the Dark Lord hoped the clones would inherit from the deceased Dark Jedi.
In the commentaries for the Special Edition, George Lucas notes that in his early conceptions of the saga, lightsabers were commonplace weapons throughout the galaxy, but that he later decided that they should be limited to the Jedi. Based on Lucas’ original conception, Ralph McQuarrie created concept art of lightsaber-wielding stormtroopers for A New Hope. This was referenced in the image above in the pose of the stormtrooper to the right of Darth Vader.