Leviathan is an organization based in Russia created for sole purpose of world domination founded by Orion. The organization serves as the main antagonist in Avengers Confidential: Black Widow and Punisher and Agent Carter.
Leviathan is a terrorist organization that was formerly a secret division of Russian intelligence. When Russia collapsed, they all went underground and spread across the globe. They expanded their power by basing themselves in economically poor regions that have been devastated by recurring wars.
We're involved in every evil deed know to man: human trafficking, assassination, terrorism, drugs...etc. They don't claim responsibility for their crimes and the objective of the organization is unknown, making them terrifying and dangerous.