Kurikara

Kurikara, also referred to as the Kouma Sword (降魔剣), is a Unique Maken created by Mephisto Pheles, then handed to the founder of the Myōō Dharani, Fukaku who used it against the Impure King and left in the possession of their sect.

It was later taken by Shiro Fujimoto to "kill a child", before inadvertently sealing its Demon Heart. It soon became the weapon of Rin Okumura as the container for his demon heart until its blade broke and his heart returned to him.

Appearance
Kurikara is a katana that has blue wrapping around its hilt, a silver handguard unusual for a katana, a silver collar above the guard, and another one on the hilt, a green tassel attached to the hilt's end. Its scabbard is lacquered colored blue with silver ornaments wrapped around it and a silver "Om" symbol on it.

After being in his possession, Rin carries around in a red sword bag on his back.

Overview
Kurikara is a unique Maken crafted by Mephisto Pheles, the King of Time, which has the uniqueness of being a "door to anywhere", its hilt leads to Gehenna and the scabbard is the door, together, it can seal demons inside of Gehenna. Due to this, it holds the power to seal demons in it, as 150 years prior, Karura known as a phoenix was for a time housed in Kurikara so Fukaku could defeat the Impure King; 15 years before the series, it was used to seal Rin's demonic heart, thus making him somewhat human, while still being half-demon.

It still maintains the power to seal despite already having Rin's demon heart sealed in it at the time, demons as it housed Ucchusma, also known as an Angi in order to use Rin's blue flames to both augment his own abilities and purify the Impure King.

Because it contained Rin's Demon Heart, unsheathing Kurikara restored his Demonic nature and powers, at least partially. Mephisto explains that this is because a Demon's heart is the source of its powers and when displaying it allows one to draw out one's true power. However, unsheathing Kurikara also leaves Rin vulnerable as it is also his vital part, thus, destroying the blade could potentially kill Rin. After learning to control his powers, Rin used Kurikara to channel his blue flames for attacks.

Despite the blade being broken, Tatsuma Suguro states that the true essence of Kurikara exist in both its hilt and scabbard, which was proven as merely using the bladeless hilt, Rin could properly control his blue flames, letting him create a blade made from flames, which allowed him to momentarily burn Satan from Yukio. In addition to it being possible for him to once again hide his demon heart inside Kurikara, letting him assume his human form.

Rin's Techniques
Since Kurikara had been in Rin's possession having been used to seal his demonic heart, it became an integral part of his fighting style since unsheathing it let him access his demonic powers and nature, letting him use his Blue Flames more than the Kurikara was sheathed. Though difficult at first, his training to use his Blue Flames even when Kurikara is sheathed has allowed him to create different techniques to use in combat.


 * Flame Emitting Meditation: A purification technique used alongside Ucchusma, who possesses Kurikara, where after uttering the correct sutra and mantra, Rin summons an orb of blue flames concentrated near the tip of his blade, which continues growing bigger and more powerful until he launches it at his opponent. Together with Ucchusma, Rin was able to purify all the miasma from the Impure King, not harming anyone or anything aside from the Impure Kings spores.
 * Satan Slash: Rin releases a slash wave of blue flames from Kurikara at his opponent.
 * Satan Bomb: Rin stabs Kurikara into the ground, then generates a series of blue fiery explosions at various nearby points of his choosing.
 * Satan Campfire: Rin stabs his opponent with Kurikara then engulfs them in blue flames.
 * Satan Bomb Campfire: Rin stabs Kurikara in his target, then causes a series of explosions at various points on the targets body before the blue flames both spread and consume the target.

Yukio's Techniques
Yukio himself is incapable of using the Blue Flames, he used them once to destroy evidence in a comical manner.


 * Evidence Destruction Flame Gun: A gag technique, which Yuki used to burn a pornographic manga that had his fingerprints on it that he initially thought was his favorite manga due to the different cover.

History
150 years ago, it was revealed that Mephisto Pheles created Kurikara then gave it to a young monk, and it would later be used to protect Kyoto from vengeful spirits who ran rampant. The founder of the sect, Fukaku, infused the Demon Karura inside the blade and with it, then brought down the Impure King around 140 years ago. However, according to legends, Karura left the sword afterward. It then acted as the sect's main object of worship.

Shiro Fujimoto was tasked by Mephisto Pheles to retrieve the sword in order to "kill a child" with it and he initially intended to steal it, but after Shiro helped cure the sect of a miasma plague, he received Kurikara from Tatsuma Suguro as thanks for his help.

Shiro used the blade, with the aid of Mephisto, to seal Rin Okumura's Demon heart into the Kurikara at birth, thus fusing it with his blue flames inherited from Satan. This let Rin stay human for 15 years until the sword could no longer contain that power after which it came into Rin's possession.

After Rin tried to save Yukio, he went berserk after being shot in the forehead, throat, and chest by him, and in response to this, Satan blocked the strike, breaking Kurikara's blade causing Rin's demon heart to return to him. Kurikara was later delivered to Tatsuma by Konekomaru, who explained that the steel could withstand the blue flames for sixteen years and needed to be preserved to reforge the blade, but he revealed that the true essence of Kurikara was both its hilt and scabbard. Kurikara is later explained by Mephisto to be a door to anywhere, with the hilt leading toward Gehenna and the scabbard being the door, meaning that together it has the power to seal demons in Gehenna thus separating Rin from his demonic nature.

Anime Exclusive
In the anime, during Rin's battle with Amaimon during the Exwire's training in the forest, Kurikara was damaged and cracked. This allowed Rin's powers to overflow and the sword was no longer able to restrain his demon heart and nature from taking over. Using nails that contained special metal from the temple destroyed on Blue Night, a female blacksmith was able to restore the sword but warned Rin that it is not perfectly restored and that it could break again with no way to fix it.

Because it contains Rin's demon heart, unsheathing Kurikara restores his Demonic nature and powers. Mephisto explains that this is because a demon's heart is the source of its powers and when displaying it allows one to draw out one's true power. However, unsheathing Kurikara also leaves Rin vulnerable as it is also his vital part, thus, destroying the blade could potentially kill Rin. After learning to control his powers, Rin uses the Kurikara to channel his blue flames for attacks.

Trivia

 * Despite being broken in his confrontation with Yukio by their biological father, Kurikara breaking didn't kill Rin as Mephisto initially said, but it could have been that if Rin's heart returned to him, his full blue flames returning to him could've resulted in his potential death.
 * Kurikara takes its name from the sword wielded by Acala in myth.