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 are intergalactic beasts and warships for the Chitauri.

Chitauri Invasion
"I'm bringing the party to you." "I -I don't see how that's a party."

- Iron Man and Black Widow

Leviathans are gargantuan beasts 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 follow him to where the rest of his team was. 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.

One of the Leviathan 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.

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

- Captain America and Iron Man to the Avengers

One 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. In 2015, during a battle in the facility between the Avengers and HYDRA, Scarlet Witch used her powers to trick Iron Man 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.

Allies

 * Thanos - Master
 * The Other †
 * Loki - Commander
 * Chitauri - Passengers

Enemies

 * Avengers
 * Iron Man
 * Captain America
 * Hulk
 * Thor
 * Hawkeye
 * Black Widow

Trivia

 * The "Leviathans" share some similarities with the Midgard Serpent, a recurring villain in Thor comics, often at Loki's service.
 * The Leviathan is also the name of the HYDRA submarine in the comic book Captain America & Thor: Avengers!.
 * Leviathans are original creations by Zak Penn and Joss Whedon.
 * In Agent Carter, Leviathan is the name of a Soviet-based evil organization.