Naginata of Kenkon

The consisted of two sharp-edged curving blades mounted on a tsuba, or hand-guard on either end of a long wooden shaft.

History
The Naginata of Kenkon was made by order of the yōkai, Hoshiyomi. Consumed by the fear of losing his beloved Tsukiyomi, (a high-level human warrior priestess who was in danger of severely overtaxing her spiritual powers and abilities due to fighting demons), he had the immensely strong and powerful two-bladed weapon created so that neither of them would have to live their lives constantly fighting and slaying other demons. In commissioning the evil demon sword-smith, Kaijinbō, to create this weapon, he was required to kill two hundred and twenty-two demons and present the essence to the smith.

Upon taking possession of the completed weapon, Hoshiyomi basked in its immense power, feeling confident that he had secured Tsukiyomi's safety. However, she came to see that the weapon was in fact evil and feared Hoshiyomi's increasingly manic behavior, having seen a terrible bloodthirsty glint in his eyes. One day, she called Hoshiyomi, and having set up an ambush, unleashed a sealing spell on him, allowing her to separate him from the Naginata. Weary, she then approached the weapon, trying to use her own incredibly strong and powerful spiritual abilities to purify and dispel the evil, but the horrifying demonic aura was too great. Thus, she split the Naginata into two halves, sending one into a far off land, to be guarded by the Hōjō clan, and the other to the bluff of Mount Hakurei.

Having expended her energies without consideration for her own safety, Tsukiyomi dies just as she attempts to release Hoshiyomi from the seal she had placed on him. An unknown number of years pass, and finally Hoshiyomi breaks free. Being distraught at her death and her perceived betrayal, Hoshiyomi resolves to regain the Naginata and take his vengeance on all humankind.

Hoshiyomi was able to obtain the Kon blade; the other half being still in the custody of Akitoki Hōjō's family. However, with the help of his demon ninjas, Hoshiyomi manages to wrest the Ken blade from Akitoki Hōjō and bring the two together to complete the Naginata. Once again in possession of the complete weapon, he clashes with Inuyasha and manages to absorb Tessaiga's yōkai energies and quell him. He then goes on a rampage to destroy the world, summoning a void to suck everything into oblivion.

Kagome, having been possessed by the spirit of the long dead priestess Tsukiyomi, takes the Naginata away from him and tells him that what he had seen as betrayal was actually her efforts to protect him from the inherent evil the Naginata of Kenkon possessed. The weapon is then destroyed, along with Hoshiyomi, by Inuyasha's Bakuryūha ("Backlash Wave"). Shortly after the destruction of the weapon by the Bakuryūha, and before he dies with this attack, he realizes his mistake after seeing a glimpse of his love one last time by the sacred power of Kagome's arrow.

Powers & Abilities

 * Heaven & Earth Manipulation: As Hoshiyomi stated, the Naginata of Kenkon was forged with the essence of 222 demons to have power over the forces of both powers: the Ken blade commands heaven, while the Kon blade controls earth. When both powers are unified, the naginata can extend its power to the max.
 * Aura: Giving off a dark purple malevolent aura, the demonic essence of the Naginata was powerful enough to corrupt and possess (as Tsukiyomi suspected) Hoshiyomi, forever changing his personality until his death. In one particular scene of note, Inuyasha is exposed to the unshielded aura of the Ken blade, while it is still in the possession of Akitoki Hōjō. In that scene, it appears that Inuyasha loses himself completely to the evil influence of the blade, unable to hear the voices of his friends calling his name. It isn't until the blade is covered again that Inuyasha returns to himself, unsure of what had just occurred. This event demonstrates the evil corrupting power of the Naginata, and shows exactly the kind of influence that Hoshiyomi has fallen under.
 * Barrier: Even with just the Kon blade, Hoshiyomi is still able to erect a yellowish barrier, exceedingly strong and powerful enough to withstand the power of Inuyasha's Kaze no Kizu.
 * Energy Absorption: At least attributed to the Kon blade, the Naginata is able to absorb both demonic energy (as in the instance with the Tessaiga) and spiritual energy (such as the instance with Kagome). In addition to this, the Naginata is able to emit this energy as a Kenatsu (see definition below), shown when Hoshiyomi fired a blast at Inuyasha right after absorbing much of the Tessaiga's demonic energy. The energy absorbed from Tessaiga was comparable to Sesshōmaru's first use of the Wind Scar. This ability is quite similar to Inuyasha's Dragon-Scaled Tessaiga.
 * Light & Darkness Blast: Stated by Hoshiyomi, the Naginata of Kenkon is the ultimate weapon of darkness and light. As such, it is able to project power blasts of light and darkness that is able to devastate its surroundings. This is shown during Hoshiyomi's first battle with Inuyasha after the Naginata absorbed Tessaiga's demonic energy.
 * Kenatsu is the special ability that allows the Naginata to cut an opponent with its yōki without physically touching them.
 * Spherical Shockwave: Similar to Sesshōmaru's spherical shockwave from his yōkai sword Tōkijin, a highly destructive sphere of demonic energy is created on the tip of the blade and dispersed in a spherical fashion. Hoshiyomi used this attack to slay a large number of yōkai in one stroke after they had surrounded him, similar to Inuyasha's Kaze no Kizu which can destroy a hundred yōkai in a single swing.
 * Nether Vortex: Quite possibly the Naginata's most powerful technique, and quite similar to the "Meidō Zangetsuha", the "nether vortex" is a form of red kenatsu burst that theoretically creates a portal to the invisible void between Heaven and Earth. This portal, however, also acts similar to a black hole in that it sucks in everything around it with irresistible force. (See also Kazaana). Being the space between the most powerful forces in the universe, it is speculated that with enough time that the black hole could absorb the entire world into itself. That would benefit Hoshiyomi's goal to destroy every human in the world and that could be the reason why he used such a dangerous technique.
 * Crimson Sphere: It is one of the Naginata's strongest offensive attacks. It takes the form of an extremely huge crimson sphere of pure energy and can be directed at an opponent. If said attack connects with someone, the sphere engulfs them and quite possibly exterminates them into nothingness. Hoshiyomi uses this as a seeming last measure attack to kill Inuyasha and his band.

Trivia

 * The arc involving this sword comes right after the third InuYasha movie, which also features a demonic sword with evil power.