Leviathan is the main antagonist of Digimon Universe: Appli Monsters. He is an evil artificial intelligence who was formerly a fragment of another a.i called Minerva until he went rouge and created a virus called the L Virus, which can infect any Appmon, and turn all humans into Appmon Chips.
Leviathan is usually a form of the symbol, which is the letter L with a serpent body. It is revealed later on, that his true form is a metallic six-headed dragon with red eyes.
When Yūjin revealed to Haru that he was actually an android created by Leviathan working for him, he summoned Deusumon to help him complete Leviathan by absorbing human victims. When Leviathan was complete, he hatched from his prison and revealed himself.
Leviathan then begin to suck the data of humans into him like a vacuum cleaner (except the AppliDrivers). Uranusmon, Poseidomon, and Hadesmon continued to fight Deusumon and defeated him. Thinking that Deusumon is useless, Leviathan sucked him in like a vacuum and crushed him with his jaws. Uranusmon, Poseidomon, and Hadesmon attacked Leviathan, but were eaten by him, much to the shock of the Applidrivers. Meanwhile, Gaiamon and Rebootmon were fighting, and grabbed hold of each other, accepting to be eaten by Leviathan.