The noxious is a variant strain of the ultralisk.
A toxic compound was bonded within the genetic sequence of the noxious. This toxin was deadly to any non-zerg entity, and could be emitted as a poisonous vapor. The noxious was also capable of channeling this toxin and releasing it as a controlled blast through vents in its carapace.
During the Battle of Korhal, Abathur created the noxious strain in the Terran Dominion's Metis xeno research facility. When Sarah Kerrigan telepathically took control of several captive ultralisks and freed them, a pair of canisters containing toxic gas broke open. The toxins, being non-lethal to the zerg, bonded with the ultralisks. Abathur used the toxins to create the noxious strain.
The noxious strain exchanges the reincarnation ability and staying power of the torrasque strain, for increased offensive potential. Its Noxious and Toxic Cloud abilities enhance its splash damage potential dramatically, allowing it to kill large numbers of enemy units faster than ever. Combined with the Monarch Blades mutation and it becomes an absolutely monstrous damage dealer to large squads of enemy units.
Release a burst of toxins around the noxious ultralisk to deal 20 damage to nearby enemies. Does no damage to structures. Can be set to autocast.
In earlier versions of the game, "noxious" was a separate upgrade for the ultralisk, explained to be a result of mutating an acidic compound in the ultralisk's lungs that is exhaled when it breathes, damaging enemies around it. The option for noxious ultralisks was initially ultralisks with the ability to burrow charge. Textures remain in the editor for a version that has glowing orange instead of glowing green detail.
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- Blizzard Entertainment. StarCraft II: Heart of the Swarm Map Editor. (Activision Blizzard) (in English). March 12, 2013