The Imperialis was a duplicate of the original opulent yacht of the same name that belonged to Emperor Palpatine. This replica was kept at the Jakku Observatory.
Raith Sienar's Republic Sienar Systems constructed the ship during the final days of the Galactic Republic.
The shipyard on Castell was the starting point for the ship's limited flight time, after which it traveled directly to Jakku. It remained there for years until the Battle of Jakku occurred in 5 ABY. Gallius Rax, the Counselor to the Empire, intended to take the Imperialis into the Unknown Regions along with former Commandant Brendol Hux, his son Armitage Hux, and his young soldiers to establish a new Empire.
However, Rae Sloane superseded him, journeying into the Unknown Regions with the Hux family and the child soldiers. A Sentinel droid, programmed to eternally obey the Emperor's commands even after death, piloted the ship. After a journey lasting several months, they met up with the flagship Eclipse of Emperor Palpatine, which had been sent ahead as part of the Emperor's posthumous Contingency plan. The occupants of the Imperialis then joined this force, forming a significant Imperial remnant dedicated to restoring the Empire.