The grammar is pretty good but the story itself is the problem. For an old man to be able to act like a 12 year old when he was always mature at a young age and then dying just to be technically tortured or training, (you hinted at what the training was like) I find it very hard to believe he's able to act like that and if you argue that the body he was transferred into has more of an emotional control then that would also be invalid because he doesn't even know simple social interactions. Plus the interactions with other is kinda awkward.

can you use POV? ok I understand if this writing style is taken from the point of view of mc or POV1 person. but you mix pov1 with narrator's POV without switching between POV 1 with narrator's POV. it's very annoying. please be more consistent. if this story only focuses on POV 1 there is no fun. to be honest in my opinion. because each storyline only focuses on what MC is living. just monotonous. I prefer narrator's POV, because in this writing style, you can tell or take a different angle, POV 1 only tells what is felt, experienced, carried out and seen. Moreover, this fanfic should be wider and not obstructed because it focuses on the MC. some original characters must also be included so that the story is better than just from the point of view of mc. thanks

This book is great. I like the old version and I like this version too, that being said title your chapters. It really annoys me when authors do not title their chapters. Other than that, I look forward to seeing more.

MC has a big mouth. he is very open and sociable. he ruined his carcass with his behavior (he would be closer to Lufy's carcass). Normal Mac (he has nothing to hook on)

Harem.Harem.Harem.Harem.Harem.Harem.Harem.HareHarem.Harem.Harem.m.Harem.Harem.Harem.Harem.Harem.Harem.Harem.Harem.HareHarem.Harem.Harem.m.Harem.

Uh Im Utterly Confused why dont you use an System Status Style Like Name:Vergil Race:Half-Human-Demon Bloodline:Sparda And His Skills Like Mana Manipulation(A)and idk why are you helping the hero but i dont like that guy

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Reveal spoiler

I honestly like it more that you send him to this anime then high school dx D because high school dx D is so overdone And in this anime the characters have actually character development. Also while it doesn't have the same power ceiling, As dxd, as shown in the anime it's still has the potential to go just as high considering the last enemy is God.