Chitauri

"The Chitauri grow restless." "Let them gird themselves. I will lead them into glorious battle."

- The Other and Loki

The Chitauri are a horse-looking faced cyborg alien race who have allied themselves with Loki.

History
The Chitauri are a powerful, technologically advanced race of alien cyborg-like creatures, and they're in Thanos' service. When the Mad Titan allies himself with the exiled god 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 fleet standing by, thanks to the help of a brainwashed Erik Selvig. The Chitauri immediately start to wreck havoc on New York City, destroying buildings and civilians in the streets. The army and the police forces aren't able to do anything about them. Soon after their arrival, anyway, The Avengers appear, and a major battle begins.

Not expecting such a resistance, the Chitauri are initially overwhelmed, but soon their number increases so much that the superheroes find some difficulties in containing them. Captain America takes command of the Avengers as well as the police forces, and thanks to his command, the Avengers' powers are properly used as well as displayed while they face the menace. Eventually, the World Security Council decides to blow up Manhattan with a nuclear bomb in order to contain Chitauri's invasion, much to Nick Fury's dislike. Fury orders Iron Man to intercept the bomb and dispose of it, and Stark takes it to the portal and blows up Chitauri's Mother Ship, thus evidently cutting the link off from the aliens' collective mind and killing the Chitauri in the immediate vicinity on the other side of the portal.

Characteristic traits
The Chitauri are a powerful warrior race, a hybrid between organic beings and machines. They're controlled by a Mother Ship which never shows on the battlefield, and acts as one during war. They have superhuman physical attributes, superior strategy and energy-based guns and flying vehicles, but their main strength is their almost infinite number of soldiers.

Relationships

 * Thanos - Master
 * Loki Laufeyson - Leader
 * The Avengers - Enemies

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

 * In the comics, Chitauri are one of the first threats faced by the Ultimates, just as in the movie, but their background story reveals that they were present on Earth even during World War II.
 * In the comics, they are a shapeshifting race kin to the Skrulls, while in the movie they appear more like cyborg-like creatures.