The Chitauri are a fictional race of extraterrestrial shapeshifters that appear in publications by Marvel Comics, specifically in Ultimate Marvel. They were created by Mark Millar and Bryan Hitch. They were created for the Ultimate universe franchise in place of the existing Marvel Comics alien species, the Skrulls, which play a similar role in the franchise's mainstream continuity. Later, Marvel would choose to distinguish between the Skrulls and Chitauri of the Ultimate universe. The species would later be adapted to other media, notably appearing in the 2012 film The Avengers as an invasion force led by Loki and even in the main Marvel universe.
The race called Chitauri appeared as Ultimate Marvel's (Earth-1610's) counterpart to the Skrulls. They are a shapeshifting alien species who have attempted to conquer the Earth, most notably during World War II and again in the early 21st century.
The Chitauri claim to be part of "the immune system of the Universe", wiping out disorder and free will wherever they find it. They seem to prefer to act behind the scenes, mimicking and influencing the social and military methods of the species they are currently infiltrating. For example, they aided the Nazis in their attempt at world conquest by providing them with the technology to create a nuclear bomb carried by an intercontinental ballistic missile. However, this attempt was thwarted by the prototype super-soldier codenamed Captain America. Through the entire conflict of World War II, the Chitauri were being driven out of their operations in Africa and Europe, even from their main Japanese training camps in Hiroshima and Nagasaki. Following the end of the war, the Chitauri withdrew to make new plans.
The next attempt at conquest was more subtle (at first), involving long-term methods of manipulation such as will-inhibiting drugs in many nations' water supplies, influencing the media, and R.F.I.D. (radio-frequency identification) microchips to be implanted in schoolchildren, among other means. The Chitauri also infiltrated S.H.I.E.L.D., particularly the Psi-Division which could telepathically ferret out Chitauri agents. However, S.H.I.E.L.D. was able to detect some of the low-ranking "drone" staff of the aliens, disguised as common office workers, and wiped them out in an assault led by Black Widow and Hawkeye.
The Chitauri planted false information through the compromised Psi-Division that led S.H.I.E.L.D. and the Ultimates into a trap on a small Micronesian island; due to the combined efforts of Iron Man and Thor, the Ultimates, Nick Fury and a handful of S.H.I.E.L.D. soldiers survived, but thousands of S.H.I.E.L.D. soldiers were killed and dozens of Helicarriers were destroyed.
The Chitauri then counterattacked by infiltrating the Triskelion and capturing The Wasp. The Chitauri leader (known only by his former Nazi identity as Herr Kleiser) took the Wasp to a hidden Chitauri base in Arizona. However, their location was given away when a fleet of damaged Chitauri starships suddenly entered Earth's atmosphere to hover above the formerly secret base, claiming that they were fleeing major defeats across the galaxy by their intergalactic enemies and that the remainder of their forces are forced to the "backwaters" of the Milky Way Galaxy (i.e. Earth's solar system). Disregarding Herr Kleiser's long efforts, the Chitauri ordered him to destroy Earth and its solar system with a doomsday bomb as part of a scorched-earth policy and retreat to the "lower fourth-dimension". The Ultimates and all available S.H.I.E.L.D. and military forces immediately converged on the alien fleet.
Although Iron Man and Thor were able to damage the ships (as well as disposing of the Chitauri bomb by teleporting it to the wastes of Nastrond, where its detonation caused only a small ripple in space-time), it was the Hulk who downed the majority of the fleet. At the same time, Captain America battled his old enemy Kleiser, but was unable to defeat him alone. At Captain America's urging/mocking of Kleiser touching Betty Ross (Hulk's crush), the Hulk was able to beat, dismember, and devour Kleiser. It is presently presumed that the combination of the Chitauri's series of intergalactic defeats and the destruction of the Chitauri fleet on Earth spelled the complete and total defeat of the alien race.
During the Chitauri-Kree War, an entire Chitauri (whose true forms resemble their Avengers counterparts) armada was fighting their enemies when they encountered Gah Lak Tus. When their ships were being heavily damaged by the entity, a rift in the fabric of reality brought a being from another universe, which called itself Galactus. Gah Lak Tus merged with the visitor, and created an even more powerful Galactus with need to feed.
The Chitauri were able to mimic human form and absorb human knowledge, apparently by ingesting the bodies or brains of the humans they imitated. In their natural form, they appear to be large, and reptilian, but no clear images of their native form have been shown. Although the drone workers were nearly mindless and not much more durable than ordinary humans, the high-ranking officers such as Kleiser possessed enough strength to rival Captain America, superhuman stamina and durability, extremely rapid regeneration, and the ability to see or sense invisible objects or people. The Hulk proves to be strong enough to destroy and consume Kleiser.
Millar's conception of the Chitauri was inspired by the British conspiracy theorist David Icke. Icke believes the world is secretly run by an elite called the Illuminati who are really shape-shifting reptilian humanoids.
Another race called Skrulls, physically resembling the Skrulls of the mainstream Marvel universe appear later (led by the billion-year-old Skrull Emperor, Kl'rt); these Skrulls dislike being confused with the Chitauri, whom they call terrorists. They have extremely advanced technology, but have not been observed to shapeshift. These Skrulls were seen only in an alternate timeline in which Reed Richards contacted their world via his teleporter. The events leading to that timeline were altered in Ultimate Fantastic Four #29 and contact was never made.
A new iteration of the Chitauri (inspired by their portrayal in The Avengers film) first appeared in the mainstream Earth-616 continuity and unlike their Earth-1610 counterpart, they are a distinct species from the Skrulls. Most of them are simple-minded dogged creatures, similar to insects, to the point of following a queen. The Chitauri first appeared as enemies of the new Nova. Nova was able to defeat an entire Chitauri armada and Titus with the Ultimate Nullifier that he stole from them in the first place. They are currently holding Nova's father Jesse as an hostage on a planet that is similar to Saturn.
The Chitauri have Jesse Alexander captive on their planet Chitauri Prime as one of the Chitauri's slaves that fight in their gladiatorial arena.
The Chitauri are revealed to have made a Chitauri clone of Jesse Alexander which also has Jesse's memories.
At the time when Captain America's memories were rewritten by Red Skull's clone using the powers of Kobik to be a Hydra agent, Red Skull's clone had Captain America steal some Chitauri eggs in order to lure the Chitauri to Earth so that they can raze it. The Alpha Flight Space Program had to fight the Chitauri swarms where they took out four different swarms in a month. A larger Chitauri swarm was defeated by Quasar where it was predicted that a much larger Chitauri swarm will appear that will be larger than the Annihilation Wave.
During the Secret Empire storyline, the Chitauri arrived at Earth where the Alpha Flight Space Program, the Ultimates, Quasar, and Hyperion of Earth-13034. As Hydra began its takeover, Captain America activated the Planetary Defense Shield trapping the Chitauri and those fighting them outside of Earth. As those trapped outside of Earth continue fighting the Chitauri, they are unable to convince the representatives on the Galactic Council into helping them against the Chitauri. As Spectrum tells Captain Marvel that they are able to make inter-dimensional supply runs and have found the location of the Chitauri Queen Eggs, the waves of Chitauri and their Leviathans are increasing in size and frequency leaving the technicians without enough time to fix the severe structural damage to the station. Monica suggests that America Chavez relocate the heroes and people stuck outside the Planetary Defense Shield to an alternate reality given the Intergalactic Council in their reality are unsympathetic to Earth's plight. Captain Marvel rejects this idea stating the heroes were charged with a mission to protect the Earth against the Chitauri invasion and they are the only thing standing in the way if the Planetary Defense Shield were to go down for some reason. Captain Marvel then turns her attention to a comatose Avril Kincaid who we find out did survive being swallowed by the Leviathan and was pulled out of the wreckage by Nova. Captain Marvel admits her fault in the situation and the raising of the Planetary Defense Shield and pleads for Avril to wake up because the team desperately needs the strength of her powers. After Quasar awakens from her coma thanks to Sam Wilson using the Cosmic Cube fragment he had in order to find a way to take down the Planetary Defense Shield, she takes down the Planetary Defense Shield allowing Captain Marvel to fly to the location of the Chitauri eggs which she destroys.