Enma is a moderately curved katana whose appearance in several ways parallels that of its sister blade, Ame no Habakiri. Its color scheme is generally dark, matching its "hell" motif (as opposed to the lightness of its counterpart). The blade possesses an irregularly flame-shaped hamon along its black bo-hi, similar to the Kitetsuswords. The tsuba (identical to Ame no Habakiri's) is trefoil-shaped, each lobe holed in the middle and thickly ridged. The tsuka is lacquered solid black (rather than wrapped in tsuka-ito), as is the saya, with both being 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.
A legendary swordsmith's masterpiece and classed as one of the 21 O Wazamono, Enma is among the best swords in the world, with Kozuki Hiyori deeming it a worthy replacement for the legendary Shusui.Tenguyama Hitetsu has described its power as being able to cut through to the "bottom of hell" (paralleling Ame no Habakiri being able to slice through "heaven"). In the hands of the master samurai Oden, Enma became the only known weapon to have ever harmed the nigh-invulnerable Kaido, giving him his only scar. According to Hitetsu, if Zoro were to succeed in making Enma a permanent black blade, Enma may increase in "rank" (位列,iretsu?), suggesting that it may become a Saijo O Wazamono.
Enma is an extremely difficult-to-wield sword, with Oden being the only one who ever tamed it. This is because of Enma's distinct quality to draw out its wielder's Ryuo (Busoshoku Haki) of its own accord, in excessive amounts when swung to force far stronger strikes than intended. Tengurama Hitesu stated that normal swordsmen or untrained individuals who use the sword will have their Haki sucked out entirely until they become dry husks. After Zoro tested out the sword, he cut a large cliff section from its base after initially trying to cut a tree. At the same time, the blade forced hardening on his whole arm and emaciated it. However, as Zoro demonstrates with enough force of will, one can force one's Haki to flow back from the blade.
Enma originally belonged to Kozuki Oden, who later used it 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 passed on as a heirloom onto Oden's daughter, Kozuki Hiyori.
After Hiyori begged Roronoa Zoro to leave his stolen Shusui in its rightful place on Wano Country, she offered Enma as a 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 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.
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.
The sword is the first object in the series to actively and directly affect the Haki of an individual.