Leviathan has 40 health and spawns in the ocean.
It always drops 
Chitin.
Leviathan is a passive ocean creature the size of a small island, listed at 40 health but immovable and indestructible in play; the only part that can be attacked is the crust of abyssal barnacles on its back, and each barnacle mined gives 3 to 4 Chitin. Every barnacle hit carries a 10 percent chance of disturbing it, and 20 seconds after the first disturbance it begins to sink, taking about 25 seconds to go fully under. Leviathans are placed at world generation with a 1 percent chance per full ocean zone and need at least 30 meters of water depth, and one that has submerged does not come back. Nothing can be built on its back.
Park the boat off its edge, mine fast, and jump the moment the shaking starts; the dive drags swimmers under. They never die and never fight back, which makes them the politest loot source in the game.
With devcommands enabled, this spawns Leviathan in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Leviathan · all console commands and IDs