The Great Leader of the Second Imperium was actually a holographic simulation of Palpatine, the deceased Dark Lord of the Sith and former Galactic Emperor. Four unidentified Imperial Royal Guards created this image to act as a symbolic leader for their newly formed empire.
The Second Imperium was established by the four Royal Guards, who used holographic projections of the deceased Sith Lord Darth Sidious, also known as Palpatine, to gain followers. These holograms depicted him at various stages of his life, from his early days as the Senator representing Naboo, to his time as Supreme Chancellor of the Galactic Republic, and finally as the Emperor of the Galactic Empire.
After conducting several raids against the New Republic, the four Royal Guards brought "Palpatine" to the Shadow Academy, housed within a large repulsorlift isolation chamber. The Royal Guards prevented Brakiss from visiting the Emperor, causing him to worry about the Emperor's well-being. Later, from within his isolation chamber, "Palpatine" encouraged Brakiss to initiate an assault on the Jedi Praxeum located on Yavin 4.
Initially, the battle proceeded according to plan. A ground force from the Second Imperium, composed of Stormtroopers and Dark Jedi, surrounded the Jedi Praxeum, while TIE fighters and TIE/sa Bombers dropped bombs on the surrounding jungle. In space, the Shadow Academy, along with a fleet of modified Star Destroyers and battle cruisers, established a blockade around Yavin 4. The situation shifted with the arrival of the New Republic fleet, led by Admiral Gial Ackbar, and Brakiss's defeat in a duel against his former Master. After returning to the space station, Brakiss went to the isolation chamber and demanded to see the Emperor. However, he was stopped by two of the Royal Guards. Consumed by rage, Brakiss killed them with his lightsaber and entered the chamber.
Inside, he found a third Royal Guard operating a console with computer screens and holographic generators, and he realized that the Emperor had never returned from death. The Royal Guards had fabricated the illusion of Darth Sidious to maintain control over the Second Imperium. Unfortunately for Brakiss, the fourth guard used the "Emperor's" override controls to activate the Shadow Academy's self-destruct system, resulting in the destruction of the Academy and the death of everyone on board. With the Shadow Academy destroyed, the battle turned in favor of the New Republic, and the remaining Imperium troops and Dark Jedi were presumably captured by the New Republic and the Imperial Remnant.