Enma is one of the 21 Great Grade swords, currently in the possession of Roronoa Zoro as one of his three blades.
As a replacement for Shusui, Kozuki Hiyori gives Enma to Zoro with the promise that Zoro will return Shusui to Ryuma's grave. After testing Enma's capabilities first hand, Zoro agrees to Hiyori's proposition. Being a match to Shusui, it's needless to say that Enma isn't just an ordinary sword either. It's been passed onto Hiyori by his father, Kozuki Oden, himself. It's also a Great Grade Sword like Wado Ichimonji and Shusui, though it isn't a black sword yet. However, it's said that once Enma's blade has been turned black, it may increase in rank and become a Supreme Grade Sword like Mihawk's Yoru and Whitebeard's Murakumogiri. Enma also has a peculiar ability. It sucks its wielder's Busoshoku Haki, which often results in an attack stronger than intended. Being wielded by Kozuki Oden, it manages to cut through Kaido's almost impenetrable defenses and leave a scar. In Zoro's hands, it manages to bring out a similar attack. Even Kaido recognizes Oden's presence from it.
The sword was crafted by the swordsmith, Shimotsuki Kozaburo in the Wano Country. Until 20 years ago, Enma was wielded by Kozuki Oden alongside his other sword, Ame no Habakiri. In Oden's hands, both blades became the only weapons known to ever (until the present day) injure Kaidou. After Oden's death, Enma was inherited by his daughter, Kozuki Hiyori, who would later gift the sword to Zoro in exchange for the latter allowing Shusui, a national treasure, to remain in Wano.
Enma is seen to be a moderately curved Shobu Zukuri-style katana with a lilac and gold color scheme. Like the Ame no Habakiri, the tsuba is trefoil-shaped, and each lobe is holed in the middle and thickly ridged. The tsuka is lacquered solid lilac (rather than cloth-wrapped), as is the saya. Both are embellished with simple, golden, 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. Like the Kitetsu swords, Enma also possesses an irregular flame-shaped hamon and black no-hi.
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.
In its initial depictions, in the A-ra-shi: Reborn video, its tsuka is black, and its hamon is golden.
As a masterpiece of the legendary swordsmith Shimotsuki Kozaburo and one of the 21 Great Grade swords, Enma is among the best swords in the world, with Kozuki Hiyori deeming it a worthy replacement for even the legendary Shusui. Kozuki Sukiyaki has described Enma as able to "cut through to the bottom of hell" (paralleling Ame no Habakiri being able to "slice heaven itself"). Kozaburo himself deemed Enma to be his best masterpiece in comparison to all his other creations, including Wado Ichimonji.
41 Years prior, at age 18, Oden used Enma, alongside Ame no Habakiri to bisect the giant Mountain God, having hardened it, and then performed Togen Shirataki. In addition to this, both swords endured clashes against the Supreme Grade Swords, Murakumogiri and Ace, wielded by Edward Newgate, and Roger. As a true testament of Enma's tremendous power and sharpness, once mastered, 20 Years prior, it became one of the sole weapons to ever harm the nigh-invulnerable Kaidou, giving him a massive scar, more so in dragon form.
Enma has the distinct trait of releasing its wielder's Busoshoku Haki, otherwise known as "Ryuo" in Wano in excessive amounts, cutting more than intended. As such, it has become infamous as a sword impossible to wield and master, so much so that no one aside from Kozuki Oden could tame Enma, whereas the Nine Red Scabbards' Leader, Kin'emon feared it. After testing it out, Zoro cut off a cliffside, despite intending to cut down a tree, because Enma forced hardening, while emaciating his right arm. However, his mastery of Busoshoku Haki allowed him to retake his Haki, something Sukiyaki commended him for, noting normal swordsmen would've been drained of all their Haki, rendering them a husk.
During the Battle on Onigashima, Kaidou had sensed a "Stange Haki" emanating from Enma, producing a flame-like aura whenever Zoro started releasing its power. It worried Big Mom, who even warned Kaidou to dodge, who noted it beared Oden's presence, causing him to recognize it as his sword. Applied on his techniques, it allowed Zoro to wound Kaidou in dragon form, leave a permanent scar on him in human-dragon form, then later deal significant damage to and defeat a Lunarian.
According to Sukiyaki, if Zoro were to succeed in making Enma a permanent black blade, it may increase in "rank", suggesting that it may become a Supreme Grade sword.
Sometime in the past, Enma was created by famous swordsmith Shimotsuki Kozaburo.
Due to its special attributes, no samurai had managed to wield Enma until it was eventually acquired by Kozuki Oden, who managed to master it as part of his Oden Two Sword Style. Oden later used Enma to wound one of the Four Emperors, Kaidou, giving him his only scar. After his death, it was somehow acquired by his father, Sukiyaki, who kept the sword safe in order to pass it on as an heirloom onto Oden's daughter, Kozuki Hiyori.
Wano Country Arc
After Hiyori begged Roronoa Zoro to leave the stolen Shusui, an O Wazamono grade sword in its rightful place as a national treasure in Wano though he earned the sword from its previous zombified owner, she offered Enma as an equal replacement. Zoro agreed to Hiyori's proposal to return Shusui for Enma, despite Kawamatsu's initial protest. Upon gaining the sword from Sukiyaki, Zoro tried it out and learned of its distinct properties, to which he accepted, and trained during the time before the raid on Onigashima to gain further mastery over Enma. During the Fire Festival, Zoro managed to gain some control over Enma's special trait, but he hasn't succeeded in mastering it. During his fight with King on Onigashima, Zoro realized that Enma was merely testing him and his suitability to wield the sword.