Testria was a terrestrial world located by Keldon Tyfus when he was a Grand Admiral for the Galactic Empire. He sent his forces there, thus avoiding the Battle of Endor all together, and established a base on the world, which later became the headquarters of his Shadow Imperium.
The planet remained unknown until the Sith Revolution, when Darth Trayus lead his forces against the Shadow Imperium's and, upon killing Darth Invisus, claimed the Shadow Imperium. Now within the sphere of influence of the new Crimson Empire, the planet retained its status as the government's headquarters.
At some point between the infiltration of the planet Wayland and the destruction of the City of the Jedi, Testria became the headquarters of the Sith Brotherhood, moved from its previous location on Tarix. The planet was also targeted by then-rogue Jedi Crystal Roshia, where a fierce duel between her and Darth Trayus and his Sith took place, whereupon at the duel's conclusion Trayus decided it be wise to move the Brotherhood's headquarters elsewhere, and thus chose its current location, Dromund Kaas.
The planet retained its status as the capital of the Crimson Empire throughout the Shadow War.