"When the powers of the Logia-type Dark Fruit, the Mythical Zoan-type Bat Fruit in its Vampire Form, and the Paramecia-type Tremor-Tremor Fruit converge into one being-" Seriously? Get real. You can't follow this sentence up with the words "no mindless power fantasy" because if the main character is in the One Piece world with three devil fruits, something that isn't known to happen even in canon (even Blackbeard only has two), then it's definitely a f*cking power fantasy.

No matter how he gains them, which is almost certainly going to be... more>> a mindless ass plot pull, having a mythical zoan, the strongest paramecia and the most broken logia, the main character is gonna be overpowered no matter what. He can null Devil Fruit abilities with the Dark Fruit, he has the ultimate destructive power with the Tremor-Tremor fruit and he's basically immortal with the Bat Fruit (assuming it's the non-canon version, and not the one wielded by Stussy.) Blackbeard got only two of those and it instantly catapulted him to the top of the One Piece power pyramid. That might have even been fine, but I'm too insulted by the author's assumption---that I'm some idiot who can't tell what's what when a lie is blatantly told to me---to even bother reading it.

Buuut, hey... Who knows, maybe they're telling the truth but it's really, really, really unlikely. The level of extreme nuance they'd have to pull off to make this not a power fantasy requires a level beyond most authors. They'd need to explain how a character gets three fruits despite no in-universe way, why the character isn't broken despite having them and how meaningful conflict, character growth, and narrative tension still exist when he's this stacked. <<less