One year Before Battle of Yavin (1 BBY) , the Blackwing virus (an infectious viral agent created through the powers of Sith alchemy) was accidentally unleashed aboard the Imperial Star Destroyer VECTOR, and later in the Imperial prison barge PURGE, turning all of the personnel into violent undeads (See the novel "Death Troopers" by Joe Schreiber). After this incident, the project was not canceled and Darth Vader sent a commando of Death Troopers to recuperate the virus into the Star Destroyer Vector and relocate it to a secret Imperial Research and Prison Facility on the remote planet Dathomir.