Hyōkotō

Hyōkotō (憑拠刀, "Cursed Sword") are a rare type of weapon which were originally invented by Mūrata Amakuni. However, the concept were later passed down to others and even altered, one example of which was the Bakkōtō. Ideally, they are the true antithesis to the Zanpakutō.

Overview
While the Zanpakutō is a symbiotic being, working and growing more powerful as the Shinigami develops a relationship with the sword's inner spirit, which is actually the manifestation and reflection of the Shinigami's very soul, the Hyōkotō is the manifestation of an evil soul that was implemented into the sword's design upon its creation. If anything, it can be called something like that of a parasite. Because of this, the Hyōkotō will only continue to become stronger and corrupt until its owner is eventually consumed by the sword mentally, driven to pure insanity, and then their soul is devoured, furthering the strength of the sword. Once this happens, the Hyōkotō can then be wielded by yet another individual who will continue the cycle. Hyōkotō do possess a will of their own, even going as far as to refuse to be handled by a person they deem weak. However, once they select a master, they can not be used by anyone else until the master that currently wields the sword has been killed and/or consumed by the sword's will. Mūrata called them the most beautiful thing, as they could become anything; solitary existences that seek power and allow nothing to deter them from that path.

Unlike Zanpakutō, Hyōkotō do not have any specific release states; instead, they evolve in proportional accordance with their owner's emotions and spiritual power.

Process of Creation
The Hyōkotō were constructed using three significant components: a Soul that contains incredible malice and evil within, the bones (or even, in some cases, the fang) of a Yōkai, and special reishi-enriched stone material found deep within Hueco Mundo. Besides this, however, the exact process is unknown.

Mūrata confessed to having personally made thirteen Hyōkotō, one of which he himself carries, while the others are in undetermined locations. Besides the, it is unknown if there exists other authentic Hyōkotō outside of the original thirteen swords.