The Sea Dragon Leviathan is a colossal, leviathan class fauna species. It is the largest aggressive creature of Subnautica. There are a total of three Sea Dragon Leviathans on the map; two in the Inactive Lava Zone and one in the Lava Lakes.
The Sea Dragon Leviathan's body mostly resembles the Sea Emperor Leviathan, albeit slightly smaller, slimmer in design, and is dark green in color, along with purple accents on its webbed limbs and head. It has protective plates on the underside of its body that start at the neck and run down to where the tentacles begin. Many of its body parts are adorned or spotted with orange bioluminescent glows, and many small spines line its lower jaw and arms.
The head of the Sea Dragon is stout and reptilian, possessing two pairs of glowing orange eyes with white pupils, a crack-like pattern spreads from the pupils to the rest of the eye. The mouth contains a pink tongue and thirty-two large teeth, visible even when the mouth is closed. The tip of the snout features two short, green protrusions with orange glowing tips. It also has a second set of larger fin-like protrusions beneath its lower jaw. Along the side of its body runs two long trails of small hollows that emit orange bioluminescent glows. It has two long forearms ending with webbed, four digited hands; each digit ends with a claw that is approximately equal in size to the player's entire body. A large sail sits on the creature's back, just below the shoulders. Its rear-side consists of seven tentacles, also lined with orange spots and ending with small tentacular clubs, the largest tentacle being surrounded by the six smaller ones.
The Sea Dragon Leviathan uses its arms and tentacles to swim in the water at great speed, according to the "Damaged Anchor Cable" entry from the Disease Research Facility, it is capable of exerting over 300 tons of force by ramming. Just being in close proximity to the Sea Dragon Leviathan will cause the player or Prawn Suit to be jostled due to the water it displaces. It will attack the player with its arms if they are on either side of its front body by swatting them away and dealing 40 damage to the player. If the Sea Dragon Leviathan attacks the player with its mouth, it will swallow the player, killing them instantly. Occasionally, the Sea Dragon Leviathan will open its mouth wide and fire a molten rock at its target as a projectile attack. The Sea Dragon Leviathan is also capable of firing a barrage of small "fireballs" covering a wide area.
The Sea Dragon Leviathan will attack the Cyclops by slamming it with its arms with considerable force, often leaving the Cyclops upside down or on its side.
The Sea Dragon Leviathan can deal massive damage to a Prawn Suit very quickly; the swat attack does 13% damage to the suit, the Sea Dragon Leviathan also has the ability to grab the vehicle in its mouth and deal 10-20% damage before releasing it; it often swats immediately after. The meteor attack can do between 24% and 48% damage to the vehicle and a sustained barrage of small fireballs can deal devastating damage to the Prawn.
When the Sea Dragon Leviathan is exposed to damage, it will flinch and commonly, if the Player is close, it will swat them away dealing large amounts of damage to the Player if they aren't in any Vehicles.
The Sea Dragon Leviathan will get stuck inside the walls of the map, but still be able to attack the player.
Spoiler alert: The following section contains story related material.
A little under a thousand years ago, a Sea Dragon Leviathan Egg was taken by the Precursors and contained within the Disease Research Facility for testing, however, the parent Sea Dragon pursued the Precursors back to the facility and laid waste to it, resulting in the Kharaa Bacterium being released onto Planet 4546B, causing the Precursors to put the planet into quarantine.
The skeleton of this Sea Dragon Leviathan can be found near the Disease Research Facility, having suffered massive head trauma from the attack.
Data Bank Entry
A colossal leviathan with reptilian features, seen stalking the very heart of the volcanic crater which underpins life in this area. The scanned specimen measured 112m in length.
Assessment: Extreme threat - Avoid in all circumstances
- The sounds of the Sea Dragon Leviathan were created by combining real sounds from a bull, an elephant and a tiger.
- Several Reaper Leviathan Skeletons can be seen in the Inactive Lava Zone; based on their data bank entry, they came to be here because of Sea Dragons that came to the surface in order to hunt, due to their primary food sources being decimated by the Kharaa Bacterium.
- The Sea Dragon Leviathan was originally larger than it is currently, the downsizing being due to a bug that was ignored for several months. The developers ultimately decided to leave it at its incorrect scale since no one remembered its original size, and also because it already had problems navigating terrain 
- The Sea Dragon Leviathan used to be able to attack the Seamoth but due to the developers figuring out that the Seamoth couldn't go so deep they removed it.