This lore and backstory was extremely well written. I can't imagine insulting or critiquing that in anyway! The combined folklore, cultish origin, and work on the anatomy is great! However, I do have some issues with the lore:
Your statement that they're comparable to frost witches in that they could be for players is great! However, the resemblance between one another is rather high with the longer jagged digits, grey skin, become cold if not catering to their nature, and thirst for blood. I don't think this is meant to knockoff Frost Witch lore in anyway; I just find it uncanny and ironic I guess to say. This isn't my big issue though.
From how this reads, one of those beasts could end up holding someone down for a prolonged period of time. And on top of that, they could make their blood flow smoother in order to siphon more blood? That just isn't relatively fair to any victim. With this very evasive nature and the brutally long claws, they shouldn't have strength WHILE also making it easier to drain more blood, ruining the victim's chances at even living if they try to run. Their blood would flow even faster, causing more to spill out, and eventually they'll faint which gives the creature the leisure of walking over to execute them finally. What makes this even worse is that you then say that:
You're going a little too far if they can have sizable strength, low claws, and evasive movement and then also be able to recover from wounds that a normal person under the influence of one of these things would most likely aim for. Their bobbing, evasive nature would most likely flail their head around where it'd be impossible to hit! You almost making them a supernatural at this point to where they resemble a werewolf. AND THEN:
Then you actually say it for me... I didn't even have to say it. Yes, they are unkillable in almost any situation it seems like. Either because of their unnatural healing rates or because no one has had the blood left to stab them. What you then suggest is that they'll retain their memories after what seems to be their death... you're asking for tons of disputes revolving around metagmaing in what they remember prior to their body that resembles their last and also how their kin will 'casually find them' in order to provide them life once more. I can see why you need this since you want to make sure that they have a chance at living since Cloud Temple Monks can't revive them. However, edit the memory recollection. Maybe to remember their backstory, not their ravenous nature before this and the people who attempted to end the creature. These are my big issues... your making a super mutant that can't be eliminated basically and has a lot of loop holes back into their existence. You're asking for tons of disputes between these monsters and players, trying to understand how 'that blow or swipe' didn't kill the beast when they were practically incapacitated by the draining blood that's faster than natural, long claws shredding at them, sizable strength to pin them down, and evasive movement to tackle them in the first place. I do want to recognize the weakness of not being able to practice the magic arts. This is a big thing to sacrifice, but that isn't big enough to become this super mutant no one can kill. Tone down the strengths a moderate amount and give more physical weaknesses.
Also, drop the sass please. That's someone's perspective on a part of LOTC that means something to them.