The Dornia Weapon is a bio-weapon created by the Dornia who communicates with Tom Mason to destroy the Espheni. According to Anne Mason, it works like a network computer virus when used on a being like the Espheni Queen who is biologically connected to her entire race. While initially fatal to humans as well, the weapon was modified to only affect the Espheni.
The Dornia communicating with Tom Mason tells him that it is working on a weapon that can wipe out the Espheni and he is the only one who can use it. However, Tom is more interested in rescuing his son from John Pope.
After the discovery of the existence of the Espheni Queen through the Shadow Plane, Tom is approached again by the Dornia who leads him to a nearby beach. There, the Dornia explains that it is the last of its kind and is trying to honor the Dornia race by destroying their enemies. It had rescued Tom as it believed he could do it. Something then shoots out of the bottom of the Dornia's nearby ship and races through the water to the beach. The Dornia reveals that the capsule inside the device sent out contains the weapon it had promised. The Dornia tells Tom to use the weapon on the Espheni Queen and the Espheni will be wiped out. Tom reluctantly takes the weapon and the Dornia disappears again. Tom brings it back to base where he is stunned to hear that his daughter has been found alive. Anne Mason and Colonel Weaver worry about how the weapon works and decide to run tests on it to see. Marty, a former biochemist, offers his help in the situation. They discover that triggering the weapon in the right place will cause tentacles to shoot out that hits the target with a grey goo. Tests on the weapon with rat test subjects determine that its a biological weapon that triggers an inflammatory response in the target's immune system and kills it. However, the weapon is so potent that if used, it could also potentially wipe out the human race. Despite Anne's suspicions of the Dornia, Cochise suggests that the Dornia simply may not have known about this effect when they created the weapon. Anne and Marty work on modifying the weapon to be non-lethal to humans. They succeed by introducing enzymes from human red blood cells and a rat that's attacked by the weapon twice survives. Marty worries that they may have made the weapon completely non-lethal, but are interrupted by Ben Mason who informs them that Tom is in danger. It turns out that Alexis is actually another Espheni clone sent to assassinate Tom. Ben hits her with the weapon which works on her. As Alexis dies, her body burns into a pile of ashes. With the success of the weapon against the Alexis clone, the 2nd Mass now have an effective weapon against the Espheni and prepare to march on Washington, D.C. to use it against the queen.
When another militia arrives, Tom shows them the weapon and explains its purpose. During the assault on Washington, D.C., Tom carries the weapon and thus has it on him when he confronts the Espheni queen alone in the ruins of the Lincoln Memorial. The queen pins Tom to a wall with a webbing and he drops the weapon which lands just out of his reach. As the queen talks to him, Tom desperately tries to reach the weapon in order to use it on her. The queen begins draining Tom of blood, but he manages to reach the weapon and takes advantage of the queen's actions to infect himself with the weapon. Due to Anne and Marty's work, Tom himself is immune to the weapon, but the virus passes through the blood the queen is taking from him and into the queen herself, infecting and killing her. As the queen is biologically connected to the entire Espheni race, the weapon's virus passes from the queen into the other Espheni, killing them. All over the Earth the Espheni explode as a result of the weapon and the Espheni are rendered extinct, ending the Invasion of Earth and freeing the planet.