Grand Marshal Crisis

The Grand Marshal Crisis was a two-year period in which an insidious figure called the Grand Marshal emerged, attacked and destroyed key New Republic bases, gaining a stranglehold on the Outer Rim, while also carving a path to Coruscant by conquering several worlds in the Inner Rim and Expansion Region, nearly reaching the Core Worlds.

Unlike other conflicts, the Marshal's aims were not to reconstruct the Galactic Empire, but rather, with the aid of ancient Sith Lord Darth Opacus, to build a new Sith Empire. In fact, a suggested name for the crisis by Republic historians was the Second Sith Crusade, but this was overruled.

Origins
The Grand Marshal also had a personal cause for starting the crisis: he had been a childhood acquaintance, Dzon Sanyon, of Luke Skywalker (although being older than him by a decade), and had felt betrayed when his T-16 Skyhopper had crashed in an unfriendly race between the two, leaving him nearly dead. Dzon thought Luke responsible for his injuries. Secondly, as an officer in the Empire, he was sorely struck by the deaths of those on the Death Star-caused by Skywalker, no less. Even though he had these motivations, he had no opportunity - until it was dropped into his lap, four years later.

Then a Commodore, he was making a report in the Imperial Palace when Palaptine's death had sparked off riots in the city. In the confusion, he broke into Palaptine's office, and found two important objects: one of Palpatine's holdout lightsabers and a Sith Holocron, made by Darth Vectivus, detailing the location of the Dark Side Pyramids. He went there and gained much power. With this power, he swayed several governers and commanders into giving him their planets, building up his strength and his military might. It is estimated he had more than two thousand ships under his command at the peak of his power.