I haven’t talked about the demons in a while so...let’s do that.
Demon “biology” physically varies from demon to demon, who each have their own unique forms save for a few exceptions (Mimics and related demons), but magically or spiritually, it’s all pretty much the same shit. Unlike mortals, they rely on magic or lifeforce to keep themselves alive, which is fuelled by food or builds up naturally (albeit extremely slowly) with rest.
In your average demon (or jinigii, as they call themselves), you’ll have one main pool that is usually located in the head, granted they have one, and then branches extending to other parts of the body. The surest way to kill a jinigii is to behead them, or otherwise separate the pool from the rest of the body (in which the pool drains out from the main artery). The more powerful the jinigii, though, the harder this is to perform. Trying to behead an extremely powerful jinigii (lets say a legionnaire) who is healthy and batte-ready will usually just end with your blade lodging an inch or so into their neck. That’s why you gotta beat the bitches down and force them to use their magic to support the body and deplete itself in the process. Then you can try a beheading.
But this is also why demon stamina is ridiculous compared to mortal stamina. A human at the peak of fitness can actively fight for about seven minutes (please don’t cite me on that tho) give or take. Demons of a sufficient power level can fight for seven hours give or take, and DO fight for that long. It’s the only way to finish the other off. Hacking off limbs is a great way to take out chunks of their life force (although the more limbs there are, the less damage you’re actually dealing (unless they’re wings as wings as energy consuming as fuck))
Legionnaires are a bit of a different story though. They have two pools because of their merging with a Kodi’ken, the second one generally forming in the centre or the meat of the body. Separating the two pools, albeit extremely hard, will kill them. Alternatively, obliterating one pool will do the same thing, meaning that they have two weak spots. It’s kind of the trade off for being that much powerful than a jinigii without a Kodi’ken.
A final note: injuring a jinigii severely while their pool is low is the only way to leave scars, as the pool can’t immediately stabilize them. This is also the only time to ensure that limbs won’t grow back.
