Enma is a moderately curved katana which has an irregular flame-shaped hamon (it has a distinct yellow-colored hamon in the music video), a golden trefoil-shaped tsuba. It has a black color scheme matching its "hell" motif. Its kashira and kojiri are both gold with its sheathe being lacquered solid black a lot like its tsuka with three green cords, both the saya and tsuka have simple light five-petaled flower motifs on both sides.
In descriptive terms, Enma is a Shobu Zukuri-style katana whose appearance in several ways parallels its counterpart, Ame no Habakiri's. It has a dark color scheme matching its "hell" motif (as opposed to the lightness of its counterpart). Like the Kitetsuswords, Enma also possesses an irregular flame-shaped golden hamon and black no-hi. The tsuba is trefoil-shaped, each lobe holed in the middle and thickly ridged (like Ame no Habakiri's). The tsuka is lacquered solid black (rather than cloth-wrapped), as is the saya. Both are embellished with simple, light, five-petaled flower motifs on each side. The kashira and kojiri are matching in appearance, being large with trefoil cutouts to match the tsuba; the kojiri possesses an additional accent, which seems to be a wide, knobbed ring above it. Three pieces of cord are present on the sword, one directly above the kashira (without hanging end), another on the saya a short bit below its mouth with two hanging, tufted ends (serving as the sageo), and yet another cord further below on the saya (again no hanging end). Further, a little bandaging is present on the tsuka for better grip.
Enma yields tremendous cutting power when fully mastered. In the hands of Oden, it became the only weapon known to ever harm the nigh-invulnerable Kaido, giving him a massive scar. 40 years ago, at 18 years old, Oden used Enma (alongside Ame no Habakiri) to cleanly bisect the giant Mountain God, having hardened it by using Busoshoku Haki. Additionally, Oden was able to use both his swords to clash against Whitebeard wielding his Saijo O Wazamono-grade Murakumogiri.
Enma has the distinctive trait of drawing out "Ryou" (Busoshoku Haki) from its wielder in excessive amounts when swung, cutting far more strongly than intended. This makes it extremely difficult to wield, so much that Oden was the only one to have ever tamed Enma. After testing the sword out for the first time, Zoro accidentally ended up cutting a large cliff section off its base after attempting to cut a tree, with Enma not only forced hardening on but emaciating his whole arm.
Hitetsu noted then that a normal swordsman trying to use Enma would be drained of all their Haki and rendered a dry husk, but as demonstrated by Zoro, with sufficient mastery of Haki, one can force the Haki extracted by Enma to flow back from the blade.
According to Hitetsu, if Zoro were to succeed in making Enma a permanent black blade, it may increase in "rank" (位列,iretsu?), suggesting that it may become a Saijo O Wazamono.
Due to its special attributes, no Samurai had managed to wield Enma until eventually it was acquired by Kozuki Oden, who managed to master it as part of his Oden Nitoryu. Oden later used Enma to wound one of the Yonko, Kaido, giving him his only scar. After his death, it was entrusted to Hitetsu for safekeeping in order to pass it on as an heirloom onto Oden's daughter, Kozuki Hiyori.
After Hiyori begged Roronoa Zoro to leave his stolen Shusui, an O Wazamono grade sword in its rightful place as a national treasure in Wano Country even though he had earned the bequeathment of the sword from its zombifiedformer owner, she offered Enma as an equal replacement. Zoro agreed to Hiyori's proposal of leaving Shusui for Enma, despite Kawamatsu's initial protest. Upon gaining the sword from Hitetsu, Zoro tried it out and learned of its properties, to which he accepted, and trained during the time before the raid on Onigashima to gain further mastery over Enma.
Enma is the king of hell (jigoku) in Japanese-Buddhist myth, the judge of dead souls. He is the Japanese version of Yama, the king of hell found in sects of Buddhism across East Asia; Enma mostly derives from the Chinese-Buddhist Yánluó, who in turn is based on the Hindu (Vedic) god of death, also called Yama. Zoro's new ownership of Enma also fitted his theme and affinity for Buddhist Spirituality as most of his sword techniques are named using Buddhist themes.
In a flashback, Oden is shown wielding a sword with a different tsuba from either Enma's or Ame no Habakiri's.
On the cover of Chapter 937, Zoro was seen reading a map featuring a sword and the word "Enma".
Enma is also the name of the shrine in a forest of Hakumai where Hiyori and Zoro end up.
Hitetsu believes one reason why Hiyori bequeathed Enma to Zoro is because of his superior strength and skills reminiscent of her father and his possession of Wado Ichimonji, which like Enma were both crafted by Shimotsuki Kozaburo.