@KamiKira00 No, it's not that he is immune to the poison. It did have effect on him and it did hurt him. But, as lazy and as BS as it sound, the author use the "devouring sword" vague description to imply that the sword could turn that poison into energy and use it on the MC.
That is also how i think the MC could "upgrade" without having to die from the lack of energy. I think that is also the reason why he keep trying to get hit so he could stack that poison. But then again, it's just what i though. I'm not saying it's not a lazy writing, but at least the author try(?) to make it seems possible
I mean, if that girl got poison AND numb/paralysis effect, then why the MC only get the poison effect?