An invasion of undead stormtroopers occurred on Dandoran at some point following the Battle of Yavin. It occurred following an outbreak at a secret Imperial research facility, where a bioweapon nicknamed the Sickness had been created. Both the Empire and its enemy, the Alliance to Restore the Republic, tried to figure out a way of stopping the undead troopers from spreading across the galaxy.
During the early years of the Galactic Civil War, the Galactic Empire maintained a secret research facility on a southern continent of Dandoran, where a bioweapons project was conducted. The facility ended up producing an infectious viral agent known to most as the Sickness.
At some point between the Battle of Yavin and the Battle of Hoth, the project went horribly awry. Imperial troops stationed at the research facility were infected by the Sickness, which killed them and turned their corpses into mindless, cannibalistic monsters. A distress call was sent, but the facility had gone silent when the Imperial command answered it. When it became evident that an army of Undead Troopers was spreading on Dandoran, commanders of the Empire tried to keep things under wraps, while the enemy Alliance to Restore the Republic tried to locate the source of the outbreak.