The Leviathan collided with the USG Ishimura while drifting through space, after which it made its way into the Hydroponics deck of the ship and barricaded itself inside the Food Storage facility. Shortly after, tracts of the Corruption began to spread at an exponential rate in close proximity to the Leviathan, and proceeded to overtake much of the deck.
Shipboard oxygen levels became critical as a result and, with the creation of poison-exuding Necromorphs known as Wheezers, the Hydroponics Deck became unable to support human life. A crew member encountered the beast and dubbed it with its name.
CEC agent Zach Hammond also encountered the Leviathan as the Hydroponics Deck lost all breathable air. Shortly after this, Isaac Clarke, having been advised about the Leviathan by Kendra Daniels, arrived on the deck, and was informed by Hammond of the creature's whereabouts. Clarke was successful in cleansing the air on the deck and also mixing a poisonous substance capable of killing the beast; however, the poison was short of truly effective, and Clarke was propelled to solve the matter by force.
Entering the Food Storage area, which the poison had granted Clarke access to by weakening the Leviathan to some extent, Clarke drew arms and killed the creature.
The Leviathan is immobile and attacks by outstretching its large tentacles to slam into Isaac and crush him. From a gaping orifice in its centre, the beast launches large organic projectiles that detonate on impact. Isaac is able to combat the Leviathan by aiming at the glowing pustules of its tentacles and a glowing yellow organ inside its central orifice. As well as using his own weapons, Isaac can also use Kinesis to hurl the Leviathan's projectiles back at it.
- It is stated within the game that the Leviathan was drifting in space until it collided with the Ishimura. If that is so, then the Leviathan obviously could not have been formed from any of the crew and is likely an entirely alien organism.