The planet Umbara, informally known as the Shadow World due to the rays of its sun never completely reaching its surface, was a perpetually dark planet within the reaches of the Ghost Nebula. It was home to the Umbarans, a species of pale Near-Humans. It was also the site of a large battle during the Clone Wars.
Umbara was part of the Galactic Republic from that government's earliest days.
The founder of the New Sith Empire, Darth Ruin, was an Umbaran and his homeworld was under Sith control by 1002 BBY, when the Brotherhood of Darkness operated a Sith academy on the world that specialized in the training of Sith assassins known as the Umbaran Shadow Assassins. These assassins used the Force to mask their presence from others, this worked even against Sith Lords and Jedi Masters. The primary weapon of these assassins was the Force pike.
During the Clone Wars, Senator Mee Deechi represented the planet in the Galactic Senate and fellow Umbaran Sly Moore worked as Supreme Chancellor Palpatine's Staff Aide. Later in the war, Umbara joined the Confederacy of Independent Systems following the assassination of Deechi and the Republic's clone troopers led an invasion to retake the planet. Since many Umbarans served as spies for the Galactic Empire, after the Battle of Endor most of them retreated back to Umbara, fearing retribution. The planet remained isolated even after the Swarm War, though many predicted the Umbarans would eventually seek to establish a presence in the Galactic Alliance Senate.