Mehlus IV was a planet occupied by both the protoss and the Zerg Swarm during the Second Great War. Abathur used the planet's hostile environment to drive the evolution of the roach. He forced generations of roaches to drink from pools containing dangerous microbes. Most of the roaches died, but those that survived formed symbiotic bonds with the microbes. Their usual acidic saliva now contained constrictive agents that slowed down enemies they attacked.
On Kerrigan's orders, Abathur created the vile strain of the roach. The vile roaches were subsequently deployed against nearby protoss forces. They successfully defeated the protoss.