The Plague of Darkness represents a disease employed by the Wyrd, a coven consisting of dark side Force witches. This plague was unleashed circa 19 BBY with the intention of attacking the ch'hala trees found on the planet of Cularin. The witches formulated this contagion utilizing knowledge passed to them by Len Markus, who spearheaded the Believers Sith cult. This knowledge originated from old Sith mutational techniques. The witches sought to corrupt the World Tree Ch'hala of Cularin, turning it to the dark side of the Force to gain control over the planet's ecosystem. Nevertheless, Jedi Master Lanius Qel-Bertuk made the ultimate sacrificed, combining his mind with the ch'hala, which allowed it to resist the plague.