Leviathans

"J.A.R.V.I.S., you ever heard the tale of ?" "I wouldn't consider him a role model."

- Iron Man and J.A.R.V.I.S.

Leviathans were intergalactic beasts and warships for the Chitauri.

Massacre of the Zehoberei
Along with the Black Order and a contingent of Chitauri soldiers, the Leviathans followed Thanos to Zen-Whoberi. There they successfully aided the Titan in his goal of slaughtering half of the Zehoberei population.

Battle of New York
"I'm bringing the party to you." "I don't see how that's a party."

- Iron Man and Black Widow

The Leviathans were used by the Chitauri during their assault on Earth as both warships and assault crafts. They roam the Sanctuary, and one such beast was straddling along the perimeter of the asteroid upon which Loki and The Other were having a discussion regarding the Tesseract. During the Battle of New York, the first Leviathan that arrived on Earth was initially confronted by Iron Man, who caused it to follow him to where the rest of his team was and was destroyed by both him and Hulk.

Other Leviathans arrived from the dimensional portal and attacked New York City, but were countered and destroyed by the Avengers. Many more Leviathans were ready to attack, but once the Chitauri Command Center was destroyed by the nuke sent in by S.H.I.E.L.D. that Iron Man diverted through the portal, every Chitauri, Leviathans included, fell to the ground and died.

Aftermath
One of the Leviathans that had fallen was photographed. The fall had killed hundreds of people. The photograph was used by the New York Bulletin, headlining its "Battle of NY" coverage. One Leviathan that was slain by Thor and Hulk fell inside Grand Central Station. Adrian Toomes' salvage company was contracted by the city of New York to salvage the technology from its corpse, but they were interrupted and ordered to hand over all the recovered salvage by the Department of Damage Control.

Scarlet Witch's Illusion
"Guys, I got Strucker." "Yeah... I got something bigger."

- Captain America and Iron Man

Leviathan corpse was recovered by S.H.I.E.L.D. and was eventually secretly stolen by Wolfgang von Strucker's HYDRA cell and placed in the HYDRA Research Base. Over the next year, Strucker and his scientists studied the corpse, obtaining several Chitauri technology pieces, especially the anti-gravity technology later incorporated into the Chitauri Armor from it.

During a battle in the facility between the Avengers and HYDRA, Scarlet Witch used her powers to trick Tony Stark into believing that the Leviathan was actually still alive and showed him visions of the dead Avengers and the Chitauri and Leviathans crossing a wormhole towards Earth, presumably to invade it.

Battle of Earth
Following the end of the Infinity War, where half-life in the universe was killed when Thanos snapped his fingers, the Avengers begun a campaign by time traveling to the past in order to recover the Infinity Stones before these were destroyed so they could be brought back to life all those who died. However, when the Thanos of 2014 was informed about this due to Nebula's overload, he captured the future self of his adoptive daughter and along the Black Order, the Outriders, the Chitauri, the Leviathans and the Sakaarans, traveled to 2023 with a new plan: destroy the universe and rebuild it on his image.

However, when it seemed that Thanos was going to win again, all of Thanos' victims came back to life and the decisive battle for the fate of the universe took place. As their fellow alien soldiers, many Chitauri warriors ended up losing their lives against the Avengers' forces. However, the battle came to an end when Thanos attempted to use the Nano Gauntlet to destroy the universe only for Iron Man to snap his fingers with the Infinity Stones he stole from the Nano Gauntlet, causing Thanos' army, including the Leviathans, to fade away into nothingness before Thanos himself turned to dust.

Characteristic Traits
The Leviathans are large creatures with serpentine bodies and massive teeth. They have are clad almost completely in armor plating that is outfitted with large fin-like blades, anti-gravity devices, and grappling hooks for Chitauri passengers to rappel from. Sone Leviathans are also outfitted with laser cannons. Much like the Chitauri, the Leviathans are linked to the Chitauri Command Center, which indicates that they also possess cybernetic implants.

Beneath their armor, Leviathans possess soft bodies that seem very frail. This can be seen when the Mark VII's well-placed missile was able to almost completely obliterate the first Leviathan's body. While it is durable, their armor proved useless against significantly powerful beings such as the Hulk, Thor, and an enlarged Ant Man.

Allies

 * Chitauri † - Passengers
 * Chitauri Gorillas †
 * Thanos † - Master
 * Black Order - Commanders
 * Ebony Maw †
 * Cull Obsidian †
 * Proxima Midnight †
 * Corvus Glaive †
 * Outriders †
 * Sakaarans †
 * The Other †
 * Loki † - Former Commander

Enemies

 * Zehoberei
 * Gamora's Mother †
 * Avengers
 * Steve Rogers/Captain America
 * Tony Stark/Iron Man † Killer
 * Thor
 * Bruce Banner/Hulk
 * Natasha Romanoff/Black Widow †
 * Clint Barton/Hawkeye
 * Sam Wilson/Falcon
 * Wanda Maximoff/Scarlet Witch
 * James Rhodes/War Machine
 * Scott Lang/Ant-Man
 * Carol Danvers/Captain Marvel
 * Brunnhilde/Valkyrie
 * Korg
 * Miek
 * Golden Tribe
 * T’Challa/Black Panther
 * Shuri
 * Dora Milaje
 * Okoye
 * Pepper Potts/Rescue
 * Erik Selvig
 * Masters of the Mystic Arts
 * Ancient One
 * Doctor Strange
 * Wong
 * Guardians of the Galaxy
 * Peter Quill/Star-Lord
 * Gamora
 * Rocket Raccoon
 * Drax the Destroyer
 * Groot
 * Nebula
 * Mantis

Trivia

 * The "Leviathans" share some similarities with the Midgard Serpent, a recurring villain in Thor comics.
 * In concept art for The Avengers, the Leviathans are listed as "Chitauri Jumbos".
 * In the film commentary, it was mentioned that "jumbo" was the creatures original name. However, it was decided to change it to Leviathan as "jumbo" would be too silly if it was made into a toy.