That's the thing you see, I have magic affinity ( alot of it ) and it still does not work. After feeding that creature at the end of the game I've racked up like 20 so im guessing its a bug
Viewing post in Leo's mod for Strive v1.0
He's right it's bugged; here's the Bloodthirster requirements:
reqs = [{reqstat = 'smaf', oper = 'gte', reqvalue = 3}, {reqstat = 'str', oper = 'gte', reqvalue = 1}],
It requires "str" but the person class only has "sstr", so the better question would by why it isn't causing constant crashing.
For DKRust, you would need to edit the save file to fix the requirements for your Bloodthirster to use it. Fixing the mod files will only fix future items.