The One Sith have spent a few centuries building their strength in the depths of the Unknown Regions. The Emperor, whom we know as Hargrev, patiently planned for the day when his Empire would repay the Galactic Federation Triumvirate for the defeat of the Sith in the past. This was same thing that happened during the era of the Old Republic when the Sith have devoted their lives to the dream of vengeance.
—Zann Skywalker in a report for the New Jedi Order
Darth Hargrev's Sith Empire, simply known as the Sith Empire or just the Empire, was a reformed One Sith government founded several years after the Second Imperial Civil War ended, with the defeat of Darth Krayt and his regime in 138 ABY. It was established on Dromund Kaas, a planet in the Unknown Regions, by Darth Hargrev, in 142 ABY. He then performed a dark side ritual on the Sith Lords who came with him to Dromund Kaas, sacrificing their life energies and greatly expanding his own lifespan. Hargrev then proclaimed himself the new Emperor, then he and his followers began rebuilding the empire.
The Sith Empire was led by the Emperor and the Council of Dark Lords, which consisted of eleven members. Each Dark Lord oversaw a certain aspect of the Empire, managing a sphere of influence and reporting to the Emperor. Under them were the Moffs and the Grand Admirals. The Imperial Military was the main organization defending the Empire, and included the Imperial Army and Navy. Their initial population grew exponentially, and more Sith loyalists eventually reached them over the years, making their numbers bigger.
After the end of the post-Second Imperial Civil War campaigns, the Sith Lord Darth Nihl and his empire were defeated by the Galactic Federation Triumvirate: the surviving New Jedi Order, the Galactic Federation of Free Alliances (its remnants), and the remnants of the Fel Empire. The Chief of State of the victorious Triumvirate, Marasiah Fel ordered the Triumvirate Navy to conduct a systematic extermination of the Sith Empire to permanently end the threat. The Jedi were against it, but allowed it to proceed, due to the popular support for it and their influence not being enough to stop it.
It was at this time that the Sith Lord Darth Hargrev, who was one of the survivors and the apprentice of Nihl, was stationed on the long-lost and forgotten Sith colony world called Dromund Kaas and began to broadcast speeches from his palace, describing how the Triumvirate would pursue the Sith and massacre the people of the Empire. There, he called all the surviving Sith Lords from Nihl's faction and promised to aid them in their conquest for revenge.
Several hundred Sith Lords answered the call, but when they arrived at Dromund Kaas, Darth Hargrev, who had learned from Vitiate (the Sith Emperor during the Old Republic era), dominated their minds and enslaved their wills to his own. Calling in ten Sith Lords, Hargrev led them in performing most powerful and complex ritual of Sith magic that he learned from the spirit of Vitiate. The result of the ritual was their life essences and power being transferred to Hargrev, greatly extending his lifespan and making him the most powerful of the remaining Sith. With that done, he declared himself to be the new Sith Emperor.
The Imperial Navy was the naval arm of the armed forces and fielded thousands of warships, logistics vessels, and smaller craft. It was under the overall command of the Admiralty, as well as individual Grand Admirals who were not part of the normal hierarchy. During the New Great Galactic War, the Navy mainly took part in engaging the Triumvirate Navy and aiding the Imperial Army in its operations. It was originally founded by Vaclen Tor, a High Admiral and veteran of the Second Imperial Civil War.