Viral spores were deadly biological weapons developed by the One Sith scientist Vul Isen during the Second Imperial Civil War as a means of pacifying rebellious worlds without committing substantial Imperial forces. They were capable of poisoning all living organisms on a planet's oceans within days, effectively rendering it uninhabitable. These viral spores were deployed in canisters and were used to great effect as part of the Final Protocol as part of the Genocide of Mon Calamari on Dac. Later, Sith–Imperial attack ships of Squadron Quad Victor deployed these during the destruction of Da Soocha.
Vul Isen examines his viral spores on Utapau.
Viral spores were a biological weapon created by the Sith scientist Vul Isen in 137 ABY, during the Second Imperial Civil War. Isen developed multiple strains of the viral-type toxin between 137 and 138 ABY, and each was capable of poisoning an entire planet. Once released into a world's waters, viral spores were immediately carried and spread by currents, killing every living thing that they encountered. Within one week, the entirety of a world, even one nearly completely covered in oceans, was affected, and nothing remained alive.
Isen's initial viral spore formulas required multiple canisters full of the toxins to be inserted into a planet's waters. However, his final strain, which took the form of a light green liquid, was potent enough that a single vial—small enough to fit inside the hand of a Human or a Givin—contained sufficient destructive power for an entire world. Isen created the toxins as a means of punishing planetary populations who earned the wrath of Darth Krayt's Galactic Empire, an organization which the scientist served. He believed that the mere threat of their use would be able to pacify rebellion against the Empire.
The different formulas were developed for use against various species, whose differing biological systems reacted uniquely to the toxins' effects. Isen's original formula targeted the population of the planet Dac, a world which teemed with a vast variety of life, and his final strain was developed for use on the planet Utapau. During the latter formula's development, scientific experiments performed on test subjects from the native Pau'an and Utai species helped Isen create a formula that would target both species. Although he was able to render himself immune to its effects, the scientist speculated that the third strain, which was adaptable, could affect any species in the galaxy.
Vul Isen developed viral spores in 137 ABY while creating experimental viruses for use on the population of the planet Dac. Darth Krayt, Galactic Emperor and Dark Lord of the Sith, ordered a genocide of the world's native Mon Calamari in that year, and Imperial forces were sent to Dac to oversee their extermination. Isen and Darth Azard, both members of Krayt's One Sith, were dispatched planetside to lead the operation. Although Krayt's orders specified that only the Mon Calamari were to die, Isen created viral spores in the hope of poisoning the entire planet and wiping out every native species. To his end, he placed the spores in strategic locations throughout the planet and waited for the order to release them. After Krayt's servant Darth Wyyrlok attempted to kill his Master and usurped the throne, the new Regent of the Empire instructed Isen to release viral spores. Isen did so, and within three days, millions of Dac's natives had succumbed to its effects. The sheer number of corpses that floated to the oceans' surfaces caused members of the starfighter unit Rogue Squadron to falsely observe from orbit that a new landmass had formed. When they realized that what they saw were bodies, they contacted their superiors within the Galactic Alliance Remnant. Alliance Admiral Gar Stazi organized a massive evacuation of Dac, and was able to ferry nearly twenty percent of the planet's various sentient species offworld before the effects of the viral spores encompassed the entire planet.
The use of viral spores to poison Dac enraged many citizens of the galaxy, among them the former Jedi Cade Skywalker. Skywalker had recently declared a personal war against the Sith, and Isen, whom the former Jedi declared the "Butcher of Dac," became his first target. Skywalker surmised that killing Isen and neutralizing his ability to create biological weapons would deal the Sith a considerable blow. While Skywalker hunted for his foe, Isen and Azard arrived at the planet Da Soocha aboard the Sith–Imperial Star Destroyer War Hammer. The Empire had learned that Azzim Anjiliac Atirue, a Hutt based out of Da Soocha's moon of Napdu, had recently given refuge to Mon Calamari refugees, and as such, Azard ordered viral spores to be inserted into Da Soocha's oceans while Azzim's spa on Napdu was bombarded by the War Hammers turbolasers. Insertion of canisters full of the toxin was performed by the Imperial bomber squadron Quad Victor.