Chitauri

The Chitauri are Loki's Army in The Avengers. They are very loosely based on the extraterrestrial race of the same name from Mark Millar's The Ultimates.

The Avengers
The Chitauri are a powerful, technologically advanced race of extraterrestrial cyborg-like creatures, and they're on Thanos' service. When the Mad Titan allies himself with the exiled Asgardian Loki Laufeyson, the Chitauri are granted to him as his personal army to conquer Earth and gain the Tesseract.

Once gained the Tesseract from S.H.I.E.L.D., Loki is able to open a portal to the Chitauri's world, thanks to the help of a brainwashed Erik Selvig. The Chitauri immediately start to wreak havoc on New York, destroying buildings and civilians in the streets. They quickly establish superiority in the city before any significant military force can respond. Soon after their arrival, The Avengers appear, and a major battle begins.

Not expecting such a resistance, the Chitauri are initially overwhelmed, but soon their numbers increase so much from the portal that the superheroes find some difficulties in containing them. Captain America takes command of the Avengers as well as the local police forces, and thanks to his command, the Avengers' powers are properly used as well as displayed while they face the menace. Hawkeye positions himself on top of a building and takes out several Chitauri flyers, Thor bottlenecks the Chitauri's advance using lightning storms, the Hulk smashes Levithians, Iron Man focuses on establishing general air superiority, and Captain America and Black Widow fight off numerous Chitauri infantry on the bridge. SWAT teams and patrol officers also fall back and hold Thirty Ninth Street against a Chitauri platoon, stalling the Chitauri advance and allowing civilians to escape.

Eventually, S.H.I.E.L.D. decides to blow up Manhattan with a tactical nuclear weapon in order to contain the Chitauri's invasion at the cost of the civilians as collateral damage there, much to Nick Fury's chargin. Despite Fury's attempts to stop it, a rogue S.H.I.E.L.D. harrier launches off the carrier and fires a small nuclear missile at the city. Iron Man chooses to bravely intercept the missile and dispose of it. Stark manages to intercept the missile and throw it through the portal, where it blows up Chitauri's fleet, including their Mother Ship, without sacrificing Manhattan to do so. This evidently cuts the link off from the aliens' collective minds and kills all the Chitauri in the immediate vicinity on the other side of the portal. Later, news channels confirm the invasion as genuinely extraterrrestrial.

Character traits
The Chitauri are a warrior race, a hybrid between organic beings and machines. They're controlled by a Mother Ship which never shows up on the battlefield, instead preferring to stay in the Chitauri's own dimension, far away from the conflict. Other than being minions to Loki, the Chitauri are never really elaborted upon, but it is known that they have superhuman physical abilities, as they are seen several times climbing up buildings. However, they still seem to be no more durable than an average human, and are seen being dispatched by a variety of weapons including bullets, blunt objects, and explosives. Notably, they seem to like getting into close combat when possible.

It is not known to what degree the Chitauri are sentient, and to what degree they are just bio-mechanical drones of Thanos. Their cybernetic enhancements, as well as the fact that they're vitally linked to the mother ship, seems to imply that they are little more than slaves. One is seen fumbling with a grenade and getting scared when it goes off in his face, meaning that they are able to react to and percieve danger like sentient beings. They also appear to scream in pain when they die, and bleed when hurt. They are as frail as humans, much like their comic book counterparts.

It's heavily implied by The Other that the Chitarui's military intelligence gravely underestimated the human forces, even discounting the presence of The Avengers.

Weaponry
The main weapons used by Chitauri infantry is a long, thin energy rifle with a bayonet on the end. These rifles, while marginally stronger than a normal human firearm, can only be fired semi-automatically. The weapon is simple to operate, and humans (namely Natasha Romanoff and Clint Barton) have been seen using it without much trouble. Chitauri infantry also appear to be equipped with armor, but not in vital spots. This leaves them vulnerable to bullets.

The Chitauri's air-superiority spacecraft are small, flying boards crewed by three Chitauri warriors. Two pilot the craft, while the third mans some sort of man-portable energy cannon mounted on his arm. This cannon has been shown blowing holes in buildings and destroying cars. While these craft are fairly fast, it is shown that the S.H.I.E.L.D. aircraft (such as the one piloted by Clint Barton) can take out several of them before going down themselves, and that they move slow enough to be hit by a bow and arrow.

Their troop transports take the form of the Leviathans, very large serpent-like creatures. These Leviathans can carry hundreds of Chitauri infantry, and are quite durable. However, they do not have any weaponry aside from simply ramming their target, and are shown to be destructible to large amounts of explosives, electricity, or kinetic force.

Relationships

 * Thanos - Master
 * The Other - Leader/ Ally
 * Loki Laufeyson - Field Commander
 * The Avengers - Enemies
 * Leviathan - Vessels

Appearances

 * Marvel Cinematic Universe (1 film)
 * The Avengers

Behind the scenes

 * Joss Whedon revealed that he chose to include the Chitauri over Skrulls or Kree because this particular race didn't have any wide background story and they could be used even for a small part.

Trivia

 * Though the Chitauri share the same name as the extraterrestrials from Mark Millar's The Ultimates, there is little resemblance between the two versions. In the comic book, the Chitauri are never shown in their true form, leaving the reader to use their imagination as to what these aliens look like. Throughout The Avengers, these Chitauri appear as tall, grey-skinned reptilian creatures. The Chitauri tended to eat humans to take their form; these savage tendencies are not included in the film. The Chitauri of the Ultimate Comics universe are shape-shifters who have been manipulating mankind for centuries, including putting the Nazi Party in power of Germany in the 1930s. Captain America encountered these aliens several times in World War II, including the mission which left him frozen in the ocean, and then fought them with the Ultimates in 2002. In the comics, they are led by a member of their own species taking the form of a Nazi Commandant named Herr Kleiser, and in the Marvel Cinematic Universe they are led by Thanos.
 * Like their comic book incarnations, they are devoted to subjugating humanity and are a scientifically advanced culture.