He has an army called the Chitauri. They are not of Asgard or any world known.
~ Thor warning about Loki's plot to bring in a Chituari invasion to Earth.
The Chitauri are an evil extraterrestrial race of shapeshifting humanoids, and a villainous faction in the Marvel comics and the major antagonists of the Marvel Cinematic Universe. They served the secondary antagonists of the 2012 film The Avengers, and the major antagonists of the 2019 film Avengers: Endgame, and as posthumous antagonists in later parts of the Marvel Cinematic Universe. They serve as both Thanos' and Loki's personal army.
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 SHIELD 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 (who as well in ridding 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.
Powers and Abilities
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.
Comparison with Skrulls
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.
Ultimate Avengers and Ultimate Avengers 2
The Chitauri appear in the 2006 animated movies Ultimate Avengers and Ultimate Avengers 2 as the main antagonists and the enemies of the Avengers and S.H.I.E.L.D. Unlike their appearance in The Ultimates, they do show their true form: seven feet tall reptilian creatures with dark greenish skin, and fire blasts from their hands.
Marvel Cinematic Universe
In the 2012 film The Avengers, the Chitauri are a race of grey-skinned, six-fingered reptilian humanoids that have a heavily cybernetic or biomechanical physiology. They are the secondary antagonists of the movie. These Chitauri display none of their shapeshifting capacities from the Ultimate universe, but have highly advanced technology, such as hovercraft-like skimmers, staves that also operate as rifles and large airborne troop carriers (known as Leviathans) that resemble biomechanical arthropods or limbless reptiles, encased in mechanized armor. They seem to have a neural link to their mothership as, when Tony Stark/Iron Man destroys the ship with a hijacked nuclear missile, the aliens seem to drop dead instantly.
They are thought to be led by one hooded Chitauri creature called "The Other" (played by Alexis Denisof). During the conversations had between the Other and Loki, the Other repeatedly mentions a powerful being later revealed in a mid-credits scene to be Thanos.
The Chitauri remained posthumous antagonists in later films.
In Iron Man 3, Tony Stark gets repeated panic attacks from flashbacks of himself leading the nuke into space and straight at the Chitauri mothership.
In Thor: The Dark World, the Chituari was mentioned as Loki was imprisoned by Odin to pay for his crimes on Earth.
In Captain America: The Winter Soldier, HYDRA used the Chitauri invasion as an excuse to build anti-alien Helicarriers, as well as using terrorist-hunting as another excuse. However, these were actually built to destroy any and all future threats to HYDRA. This leads to S.H.I.E.L.D. being disbanded and rebuilt, due to HYDRA being inside S.H.I.E.L.D. for decades. In the mid-credits scene, we see that a HYDRA agent named Wolfgang von Strucker is collecting things of power; two prisoners are shown, Quicksilver and Scarlet Witch, while one of these items shown is Loki's mind-control staff, which was a Chitauri weapon.
In Avengers: Age of Ultron, the Chituari were shown in a vision by Thor as he relates to a certain being (which was revealed to be Thanos) manipulating the events for his own benefit, just as Ultron stole Loki's staff (which contains an Infinity Stone) for his plot to destroy mankind with his army of Sentinels.
A strong level of the Chitauri's legacy is presented several times in the TV series Marvel's Agents of S.H.I.E.L.D. When Coulson was brought back to life using the blood of a Kree alien, a hacker group called the Rising Tide set in and proclaimed that the secrets are out, aliens are real and S.H.I.E.L.D. has been lying to the world about many things. In the pilot episode, a S.H.I.E.L.D. agent/HYDRA double-agent named Grant Ward goes to an arms-dealer's home in Paris and manages to steal a "Chitauri neural link". What it does is unknown, but it might have been why they all died when the mothership was nuked in The Avengers. In the episode "F.Z.Z.T", several firefighters were sent to New York after the invasion, with one fireman from a small Pennsylvanian town taking a Chitauri helmet home as a souvenir. However, it housed an alien virus that spreads by static shock and slowly killed the firefighters who caught the shock; an explosion of light would occur, and the body would be floating in mid-air. In the episode "The Magical Place", it is implied that salvaged Chitauri metal was used in the technology for the Centipede Serum seen several times throughout the series.
The Other makes another appearance in Guardians of the Galaxy, but Ronan the Accuser breaks his neck. A Chitauri prisoner was shown later in the movie among the collected items of Taneleer Tivan's museum.
Chitauri weaponry is a major plot device in Spider-Man: Homecoming. Inventor Adrian Toomes used a cache of Chitauri tech he and crew appropriated from after the invasion of New York and reverse-engineer to construct the Vulture suit, Shocker's gauntlets, as well as Anti-Gravs and laser guns. Spider-Man had found a Chitauri gun power core while chasing Shocker down. The core later caused an elevator accident at the Washington Monument that Spider-Man narrowly saved its passengers from. A malfunctioning Chitauri gun during a fight between Spider-Man and Vulture on the Staton Island Ferry unleashed beams of energy that slice the hull in half, which Spider-Man fails to hold together, but Iron Man narrowly saves.
The Chituari made a cameo appearance in Avengers: Infinity War, where they are seen in a flashback aiding Thanos and Maw in slaughtering the Zehoberei people as he takes one of their children Gamora as his own. A few Chitauri were later seen on Thanos' flagship Sanctuary II, acting as guards and tortures for Nebula. Nebula killed one of the Chitauri while escaping.
The Chitauri appear as part of Thanos's army during the final battle in Avengers Headquarters in Avengers: Endgame. Iron Man uses the power of the Infinity Stones to exterminate all of them.
Their role in The Avengers has led to many events happening in the Marvel Cinematic Universe as without the Chitauri invasion, many major events in the franchise wouldn't have occurred, like the HYDRA's rise to power or the Vulture's campaign against Tony Stark.