Enma as a sword is immensely powerful. Its blade is as sharp and as well crafted as any other O Wazamono blades like the Wado Ichimonji. However, Enma’s special ability is incredibly strange.
Where did this come from? What exactly does it do and how does it work?
1. Where did its ability come from?
Nothing in the series has given an answer to Enma’s unique ability. However, Wano has shown us many unique and strange blades. Personally, I hope this is a curse so that it may relate to what a cursed blade is or discussion on how blades are cursed.
Canonically there are 3 known cursed blades in Wano. Sandai Kitetsu, Nidai Kitetsu and Kikoku.
There have also been other swords that are stated to have unique powers like Kiku’s that cuts hurt even in death and Enma that drains and uses its user’s Haki. So I hope it’s a curse so that that may be expanded on.
2. What exactly does it do and how does it work?
Honestly the best way to describe its power is by a quote from another anime: “Let’s say this lake was the magical powers of one of the Seven Deadly Sins. How much water can you scoop out just using your hand? See no matter how much magic you may have, this is all your hand can hold. But with a weapon, it’s like using a glass or a beer much. You can extract a lot more magic.”
This is a quote from King from Nanatsu No Taizai (7 Deadly Sins) and I believe it works with Enma very well.
Someone’s Haki pool may be immense and deep; however, the amount someone can bring out at once is a whole different kind of training.
What Enma does is it basically allows Zoro to pull out that entire Pool of Haki at once. With training, Zoro is able to lower the amount of Haki it pulls out of the pool; however, with Enma, the amount of Haki Zoro can pull out of his pool in one or two attacks, dwarfs the amount he could be able to pull out without Enma.
So yes, Enma uses Zoro’s own power; but it allows Zoro to use more of his own power than he could without it.