Chitauri

"The Chitauri grow restless. Let them gird themselves. I will lead them into glorious battle. Battle? Against the meager might of Earth? Glorious, not lengthy. If your force is as formidable as you claim.""

- The Other and Loki

The Chitauri are a sentient species of cybernetically enhanced beings operating under a hive mind intelligence. Subservient to the overlord Thanos, they were most notable for being the first major threat to Earth that required the formation of the Avengers when they attempted to commence a planetary invasion as part of an alliance between Thanos and the Asgardian Loki.

History
"He has an army called the Chitauri. They are not of Asgard or any world known."

- Thor

When the warlord Thanos allied himself with the exiled Loki, the Chitauri were granted to him as his personal army to conquer Earth and gain the Tesseract.

Once having gained the Tesseract from S.H.I.E.L.D., Loki was able to open a portal to Earth, with the Chitauri fleet standing by, thanks to the help of a brainwashed Erik Selvig. The Chitauri immediately started to wreak havoc on New York City, destroying buildings and gunning down civilians in the streets. The National Guard could not deploy enough troops quickly enough to counter the invasion. Luckily, the Avengers arrived, having fought Loki repeatedly in an attempt to hinder his efforts at summoning the Chitauri. Soon after their arrival, a major battle ensued between them and the Chitauri.

Not expecting such a resistance, the Chitauri were initially overwhelmed, but soon their numbers increased so much that the superheroes found some difficulties in containing them. Captain America took command of the Avengers as well as the police forces, and thanks to his command, the Avengers' powers were properly used while they fought the menace. Eventually, the World Security Council attempted to destroy Manhattan with a nuclear bomb in order to contain the Chitauri invasion, but Iron Man intercepted the bomb and disposed of it into the portal, where it impacted and obliterated the Chitauri Command Center, instantaneously disabling all Chitauri forces on Earth, thus ending the invasion.

After their defeat, pieces of Chitauri technology were recovered by both S.H.I.E.L.D. and various other parties.

A Chitauri was one of the specimens being held in Taneleer Tivan's museum before its explosion.

Later, fragments of Chitauri technology would be used by the remainder of HYDRA and Hammer Industries.

Culture and society
The Chitauri are a powerful, reptilian warrior race, a hybrid between organic beings and machines. They have a caste-like society, with each caste (nearly a different species in itself) fulfilling a different role in Chitauri society. They are controlled by a Mother Ship which never shows on the battlefield, and act as one during war. They have superhuman physical attributes, superior strategy, energy-based rifles and flying vehicles, but their main strength is in their great numbers.