Sith plague

"Tenebrae's final gruesome weapon."

- Scourge on the plague

An ancient Sith plague was unleashed concurrently with the Battle of Zakuul. When the Commander of the Eternal Alliance cast the spirit of the Immortal Emperor Tenebrae out of his mind, the former Emperor's Wrath Scourge and the Jedi Knight Kira Carsen found and destroyed his original Sith body. This triggered a trap laid by Tenebrae: the destruction of this body was tied to a Sith ritual that released the plague into the air. Since Scourge and Carsen had already been affected similarly by the emperor in the past, they were not infected, but left comatose for more than a year.

Jedi Master Satele Shan led a mission to rescue Scourge and Carsen around 3630 BBY. The plague still lingered, however, and Shan and her followers were unknowingly infected. Within days, they were left in a nightmarish sleep, in which they slowly began merging into a hive mind. Scourge and Carsen, having been successfully rescued by the Jedi, captured them and kept them quarantined in a transport vessel flying an undetermined course through unsettled sectors of the Outer Rim Territories.

Characteristics
"The infection is worsening. There is a darkness growing among the afflicted."

- Scourge

The plague's symptoms were apparently purely psychological, with no physical changes mentioned. Symptoms included coma, nightmares, and delusion (the afflicted being lost in a dark side hive mind). Even individuals very strong in the Force were easily clouded by the plague. Individuals who had previously been affected by Tenebrae in similar ways, such as Children of the Emperor or members of his power base, were immune to the disease, only being left comatose upon contact.