Dakki

is a yōkai sword that absorbs demonic energy. It was from absorbing Dakki's technique that the Dragon-Scaled Tessaiga was created.

History
It was created by a sword-smith named Tōshū, who wanted to create the strongest blade around. Through experimentation and creation of many blades, he found that blades of stronger yin yield better results. He started to collect broken swords from areas of conflict and despair. These acts soon earned the attention of the demon Ryūjin, who was drawn to Tōshū's yin and thus commissioned him (by Tōshū's request) to create Dakki.

In the manga and the anime (InuYasha: the Final Act), Inuyasha's group came to know about it after a raccoon demon, who tricked Shippō into helping him, fought with Inuyasha to steal the demonic energy with Dakki. However this version was not the true version, as it easily broke from a simple knock of Inuyasha's sword. They took a piece of the broken blade to Tōtōsai, who examined it and told them about the sword-smith who forged it. When Inuyasha decided to find Dakki in order to upgrade Tessaiga, Tōtōsai warned him not to be overconfident, as even the fake version was able to steal demonic energy quite easily, should Inuyasha have to battle the true Dakki, he risked having Tessiaga devoid of its demonic energy and be in its battered form forever.

Later they rescued Tōshū from Ryūjin, who was angered that Tōshū had not yet completed crafting Dakki. When Ryūjin was defeated by Inuyasha with his Backlash Wave, Tōshū used the sword to absorb Ryūjin's power, thus completing Dakki and gaining the Dragon Scales (In the manga, Dakki already has the dragon scales, and merely gained Ryūjin's demon power.), then battled Inuyasha and began to steal the demonic energy from Tessaiga. Through the course of the fight, Tessaiga began to crack as a result of having its energy stolen, while Tōshū began to turn into a demon himself. Soon Tessaiga turned back into its battered form due to the massive loss of energy, but not without finally making a small crack on Dakki. Despite the overwhelming sword skills of Tōshū under the influence of Dakki, Inuyasha still battles him alone, as he believed it to be a fight between Dakki and Tessaiga, and he did not want to waste the opportunity provided by it to destroy Dakki.

Eventually Dakki, sensing the danger of getting destroyed, used the demonic energy taken from Tessaiga to kill Tōshū, leaving behind only his hand attached to the hilt. Shortly after the blade broke from the crack, releasing all the stolen energy which returned to Tessaiga, followed by the ability to absorb demonic energy.

General Appearance
It takes the form of a large katana which, after Tōshū killed Ryūjin and absorbing his powers, gains dragon scales on the backside of the blade (In the manga, Dakki gained the dragon scales upon being forged.). If a human were to wield the katana, the continuous absorption will corrupt that person, while changing him into a demon, with scales appearing on his hands (where they are gripping the hilt) and extending to his body. A yōkai wielding the sword would be able to absorb energy to the extent that their own Yōki can restrain it, same with a hanyō.

Abilities
Yōki Absorption: It has the ability to absorb demonic energy from demons and their attacks. The fake version was able to steal the demonic energy from Tessaiga from its attack, while the true version was able to steal large quantities by merely touching the Tessaiga. 夺鬼