Little is known of this mysterious organization. It was born out of the Communist Bloc in the same way that Hydra was born out of the Axis powers (the Third Reich, the Italian Social Republic and Imperial Japan), but they have given nothing more away.

What is known is that 100,000 of the Eastern Bloc's finest Cold War agents simply disappeared one day. In truth, they had been taken to the facility known as Cold Storage, where the majority were placed in cryonic suspension until they were needed. Apparently, the higher ups of Leviathan were involved in a collaborative game with the leaders of Hydra, S.H.I.E.L.D. and the Hand set by Aries. They gathered together alien technology from the Brood and a collection of tanks. Together, these devices could turn a man into a Super Soldier. Leviathan then chose to betray their partners and take the technology for themselves. Hydra and the Hand struck back, stealing the alien power source, making it so that the tanks would turn their occupants into monsters.

During the Dark Reign, when S.H.I.E.L.D. had been disbanded in favor of H.A.M.M.E.R., and Hydra was tied up fighting the renegade Nick Fury, Leviathan decided to strike, dealing Hydra a massive blow.

This became a massive three-sided war between Hydra, Leviathan, and Nick Fury and the Howling Commandos PMC, with each side suffering substantial losses. The Howling Commandos have been all but wiped out, while both Hydra and Leviathan had been substantially reduced in size, with relatively few operating agents remaining on either side. Nick Fury contributed to Leviathan's defeat by remotely tapping into the activation systems that brought the Leviathan's soldiers out of hibernation, feeding them so much energy to ensure that only they would have a short lifespan. Only Orion remained.