(Some world-building for Blades in the Dark.)
The first instinct is to make leviathans whales, albeit demon whales. So they are slain and harvested, right? But that is not John Harper’s view. They are truly immortal; they cannot be slain, so harvesting their blood is done mosquito-style, until the leviathan breaks free. This changes things.
Maybe you still like having leviathan bone in your game, and blubber, and whale-skin leather, and so on. If you do, I have a way you can get there.
Before the industrialization of the leviathan blood really took off, the hunting was far less regulated. One of the most successful methods was pioneered by cultists who learned the lure for the leviathans from their demon consorts.
If the leviathan was within a certain distance of an appropriate beach, you could sacrifice humans in a proscribed ritual. Every dozen increased the range and power of the lure, topping out somewhere around 108 sacrifices.
The leviathan was drawn inexorably to the flowing blood of the ritual murders, plowing up onto the shore, gasping and beached. While it was there, you could get a good look at it.
Every leviathan’s propulsion is a bit different. There is a mix of flippers, whip tendrils, stubby legs, and other limbs. A specialized team of insane butchers would approach the beached leviathan and target one or more limbs, removing them with massive saws, blades, drag harnesses, and other equipment inspired by a combination of butchery and lumberjacking. One massive flipper might have a hundred pounds of bone in it. Leviathans stripped of one or more limbs could grow them back in time, so the resource was renewable.
This practice came to be known as limming, practiced by limmers. (The “b” of “limb” was dropped because it made pronunciation confusing.)
Now that industrial production requires harvested leviathan oil, and major companies (that are publicly acceptable) hunt the giants, the practice of limming is considered a distasteful relic of the past. Human sacrifice is not legal. Luring leviathans to the shore is irresponsible and dangerous. There is no need for leviathan meat or bone. Standard hunting techniques of the massive metal ships can garner some blubber, leather, and the necessary blood.
Still, the dark practice of limming is responsible for most of the whispers in the city; the fury of the leviathans in their weird blood changed those who tortured them at point-blank range and lived. Cultists carve demonic insights on leviathan bone, dreams processed through the human psyche and desperate to return to the demonic source. Get deep enough in the criminal world, or the religious world, and you’ll find limming is not quite as extinct as the civilized world pretends.