The Meidō Zangetsuha (冥道残月破, "Dark Path of the Dawn Moon's Wave") is an immensely strong offensive technique that creates a path that sends anyone who is hit with it directly to the Netherworld itself, killing them without actually harming them. The technique originally belonged to the yōkai Shishinki. The Inu no Taishō battled Shishinki and defeated him with the Tessaiga and used it to steal the technique. Because of the technique's dark nature it was cast off Tessaiga and Tenseiga was created to contain it. Tenseiga was then given to Sesshōmaru as he was the only one capable of perfecting the technique while withstanding the terrible jaki associated with it, until Inuyasha was ready to utilize it.
The Meidō as it was originally with Shishinki resembled a black sphere, but Sesshōmaru's version started out with a crescent shape, eventually gaining an elliptical form as Sesshōmaru gained strength and experience. However, he was incapable of recreating the Meidō in its original form because of Tenseiga's nature as an incomplete sword, until the sword resonated with Tessaiga during his encounter with Shishinki, enabling Sesshōmaru to perfect the technique while manifesting a Meidō bigger and stronger than Shishinki's version.
Later, Inuyasha would prove himself worthy as the true wielder of Tessaiga and Meidō Zangetsuha in Sesshōmaru's eyes during their final battle, and Sesshōmaru willingly relinquished the perfected technique to Inuyasha by breaking Tenseiga over Tessaiga, allowing the technique to be absorbed back into Tessaiga and giving the sword a new alternate form in the guise of a black blade resembling a Meidō.
The technique would undergo one final transformation to better reflect Inuyasha's ownership of it as well as Tessaiga's nature as "a sword that cuts". The gigantic sphere was replaced by a barrage of blades that ripped apart whatever they hit and sucked up the remains, making it more effective against larger and more powerful opponents. In this new form the Meidō finally started to work on Naraku, who proved to be resistant to the attack's original form. Only the Shikon Jewel itself was able to survive this technique's full power and strength.
In the manga, but not in the anime version, Magatsuhi and Naraku were destroyed forever by Inuyasha when he cuts the point of light in the darkness inside the Shikon Jewel with Meidō Zangetsuha, which is the key left by Kikyo before dying to kill Naraku and destroy the jewelry evil. In fact, with Inuyasha's attack at this very point, the purity of the Jewel called Naohi annihilates the evil corrupting presence within it, then the Jewel's shimmering and voice completely stopped.