To the people talking about the knife, remember the Ouroboros had roughly 10x the stats as our MC, so if he was possessing Helen it's very possible that her attack stat now outmatched the MC's defense, allowing even a regular knife to cut him. Heck, if you note, at the end of the chapter Helen has the powers of the dragon as she's able to transform into it, meaning that while she was possessed she did indeed have the stats of the dragon thus her attack (despite the knife adding basically nothing to it) was able to overcome the MC's defense, thus no knife break.