The Emperor's special reserve guard was a group of elite starfighter pilots that were presumably directly under the command of Emperor Palpatine himself and placed under reserve.
Two of the members of traitorous Grand Admiral Demetrius Zaarin's fleet, pilots of Mu Squadron (Z-Mu Squadron), that attacked Palpatine's C-3 passenger liner Excalibur were also members of the Emperor's special reserve guard, which was discovered by Imperial Intelligence shortly after the attack was quelled by Maarek Stele. This discovery led to the realization that there were spies of Zaarin that had penetrated even the very inner workings of the Imperial Military hierarchy.