Lethal biological weapons, known as viral spores, were the creation of One Sith scientist Vul Isen during the Second Imperial Civil War. Their purpose was to subdue rebellious planets without the need for large Imperial military deployments. Within a matter of days, they could contaminate all marine life on a planet, rendering it uninhabitable. Deployed in canisters, these spores proved highly effective as part of the Final Protocol during the Genocide of Mon Calamari on Dac. Later, Sith–Imperial attack ships from Squadron Quad Victor unleashed them during the destruction of Da Soocha.
During the Second Imperial Civil War in 137 ABY, the Sith scientist Vul Isen invented a biological weapon called viral spores. Between 137 and 138 ABY, Isen engineered several versions of the viral toxin, each capable of poisoning an entire planet. Once introduced into a world's waters, the spores were rapidly dispersed by currents, killing any living organism they encountered. Within a single week, the entire planet, even those predominantly covered by oceans, would be affected, leaving no life behind.
The early viral spore formulas required multiple canisters of the toxins to be released into a planet's waters. However, Isen's final version, a light green liquid, was so potent that a single vial—small enough to be held in the hand of a Human or Givin—contained enough destructive power to devastate an entire world. Isen developed these toxins to punish planetary populations who had incurred the wrath of Darth Krayt's Galactic Empire, the organization he served. He believed that the mere threat of their use would quell any rebellion against the Empire.
Different formulas were designed to target various species, as their unique biological systems reacted differently to the toxins. Isen's original formula targeted the inhabitants of Dac, a planet brimming with diverse life forms. His final strain was intended for use on Utapau. During the development of the latter formula, experiments on native Pau'an and Utai test subjects aided Isen in creating a formula that would affect both species. Although he managed to immunize himself against its effects, the scientist speculated that the adaptable third strain could potentially affect any species in the galaxy.
In 137 ABY, Vul Isen was developing experimental viruses for use against the population of Dac when he invented viral spores. Darth Krayt, the Galactic Emperor and Dark Lord of the Sith, ordered a genocide of the native Mon Calamari that year, dispatching Imperial forces to Dac to oversee their extermination. Isen and Darth Azard, both members of Krayt's One Sith, were sent to the planet to lead the operation. Despite Krayt's orders to only kill the Mon Calamari, Isen created viral spores with the intention of poisoning the entire planet and eradicating every native species. To achieve this, he strategically placed the spores throughout the planet, awaiting the order to release them. After Krayt's servant Darth Wyyrlok attempted to assassinate his Master and seize the throne, the new Regent of the Empire instructed Isen to release the viral spores. Isen complied, and within three days, millions of Dac's inhabitants had succumbed to the effects. The sheer number of corpses floating on the ocean's surface led members of the starfighter unit Rogue Squadron to mistakenly believe from orbit that a new landmass had formed. Upon realizing that it was actually bodies, they contacted their superiors within the Galactic Alliance Remnant. Admiral Gar Stazi of the Alliance organized a large-scale evacuation of Dac, managing to transport nearly twenty percent of the planet's various sentient species off-world before the viral spores consumed the entire planet.
The use of viral spores to poison Dac sparked outrage among many citizens of the galaxy, including the former Jedi Cade Skywalker. Skywalker, who had recently declared war on the Sith, made Isen his first target, labeling him the "Butcher of Dac." Skywalker believed that killing Isen and eliminating his ability to create biological weapons would inflict a significant blow to the Sith. While Skywalker hunted his enemy, Isen and Azard arrived at Da Soocha aboard the Sith–Imperial Star Destroyer War Hammer. The Empire had learned that Azzim Anjiliac Atirue, a Hutt operating from Da Soocha's moon of Napdu, had recently provided refuge to Mon Calamari refugees. Consequently, Azard ordered viral spores to be released into Da Soocha's oceans while Azzim's spa on Napdu was bombarded by the War Hammer's turbolasers. Quad Victor, an Imperial bomber squadron, carried out the deployment of the toxin-filled canisters.