They are primary antagonists of N.O.V.A. 1.
When a N.O.V.A ship crashed on the Xenos mobile asteroid, the automated command on the asteroid mistook it as a threat and promptly sent the Caretakers to wipe out the race responsible: Humanity. Numerous engagements took place around the human empire, with humanity losing slowly. When Kal Wardin was teleported into their capital, however, and destroyed their automated command, Those Who Judge woke and lectured Kal, saying how some humans are dishonest and greedy and those must be destroyed, while the majority will be spared largely through Kal's action. They also revived Yelena and dubbed Prometheus the protector of humanity. It can be assumed that the war with the Xenos soon ended.
The Xenos played a much smaller role in N.O.V.A. 2. During the level "The Ring of the Apocalypse", the Alliance forces and Volterites attempted to steal the Judgers' doomsday weapon, also called the Ring of Apocalypse. The weapon was hidden in a small Xeno temple on the planet Scorpius. The temple had a self-defence system that dispatched troops to kill any non-Xeno life form. Fortunately, the Alliance forces and Volterites were all killed by the Heavy Demons and Corrupted Marines. Unfortunately, they also tried to kill Kal Wardin. This attack on the temple by the alliance was just a distraction so they could find a different artifact on the planet Sagitarius.
The Xeno were primarily of two main races, The Judgers and the Protectors (The protectors are the enemies mentioned below) The Protectors were composed of 6 different races. The Protectors are not very intelligent. They are biologically engineered and utilize poweful energy shield armor that protects them from bullets to toxic waste. The Judgers are very intelligent spiritual beings and they control the Protectors.
Powerful, ape-like Xenos who carried two large Clubs. They are notorious for their ability to grab N.O.V.A. Troopers, lift them up, and attempt to bash them with their Clubs. They will throw their Clubs at long range and attempt to grab their opponents when they have thrown both Clubs. A more powerful Yellow version appeared in the Judger City.
Large, somewhat reptilian, Xenos who possessed the ability to launch Fireballs out of their hands, they would smack close range enemies and pick up explosive objects to hurl at their opponents. After throwing several Fireballs, they will release a wave of 5 Green Fireballs.
Reptilian, armored troops that wielded projectile Particle Rifles, they would also throw Grenades and utilize stationary machine guns. They serve as the basic infantry of the Xenos. There are also different red Grunts who use jetpacks. Their are also green ones who are generally stronger or better fighters.
Small reptilian-insectoid creatures who ran on all-fours, they had the notable ability to hide in walls and ambush their prey. They typically jump and slashed at their prey. Later renamed Spiders in N.O.V.A. 3.
Large, heavy creatures armed with twin Incinerator Cannons who could Teleport. They were mostly immobile, but their Cannons were extremely dangerous.
Small, Insectoid creatures who would rip open the armor of a N.O.V.A. Marine and implant themselves in the chests of their victims. Although the parasites don't attack you directly their victims do and they are capable of using Shotguns, Assault Rifles, Pistols, and Sniper Rifles.