The Emperor's Reach represented an elevated rank of Dark Side Adept who were devoted to Emperor Palpatine—also referred to as Darth Sidious, who held the title of Dark Lord of the Sith. Their special capability was using the Force to communicate over vast stretches of space.
The Emperor's Reach existed as a role analogous to other positions such as the Emperor's Hands, the Emperor's Eyes, and the Emperor's Voice. The Prophets of the Dark Side were tasked by Palpatine with training one member, who had formerly belonged to their group.
Maarek Stele held membership within this clandestine group, having been selected from the ranks of the Imperial Navy.