"...Yeah, they went that way and they're still close enough that if you called out for them they'd hear you," Sarah says.

Spoiler: The Legend of The Maken
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Once Upon a time, there was a blacksmith who served along an army of thieves and murderers. He created many, many swords of great quality with which this army enforced its will by any means. One day, he stumbled across the means by which the stuff of souls could be made into metal and forged into swords greater than any made of mere iron.

He found a great and evil spirit and forged it into a mighty blade to be wielded by one worthy of being king.

He forged four blades emblematic of the Noble Houses that is army secretly served.

He created a harem of living swords who could appear as beautiful women.

And he created a process by which fragments of souls imbued with the properties of the souls of the Gods of Death, who composed the army of thieves and murderers, and forge them into "empty" swords that bond with the soul of the wielder and become one with it, efficiently manifesting the native powers of the soul and awakening within it a manifestation of one's true self, a feat that earned him a place forging swords in the realm beyond.

However, with every great art there comes... Imitators. Centuries ago, a blacksmith who studied the work of the earlier smith attempted to replicate and improve upon the design of the shallow hilts by combining them with the first and greatest of the Soul Cutting Swords, merging not only the shards of Shinigami souls(created... The Hard Way) but also those of hollows and, by some twisted sorcery, that of a single still-living human and a single still living Quincy. None of those beings still exist, they are merely raw materials used to create a perfect sword. A Magic Sword. A Demonic Sword.

The Maken(魔剣)

The swordsmith who created it, for having violated the Laws of the Afterlife and pursued illegal means of research and forbidden magics in his creation of the blade, was pursed by the authorities of Soul Society for his crimes and when confronted in his forge without his personal weapon tried to defend himself using his prototype. This was a mistake.

In theory, the Maken is an ordinary Asauchi other than that it would enhance the powers it draws out an allow them to manifest in a manner that is more instinctive than otherwise. In practice, combining the Ausauchi's ability to bond with and merge with the souls of their wielder with the raw mutability of human souls, the... absorptive and manipulative properties of Quincy power and, most importantly, the infectious, corruption, and assimilative nature of hollows resulted in a blade that didn't adapt to it's hosts soul by devoured it, broke it apart, and assimilated it.

The Foolish Swordsmith was driven into a murderous rage, slaughtering all but one of his would-be captors and devouring their corpses in a manner not unlike a hollow before breaking apart into raw reishi and being absorbed into the blade within seconds. The blade was taken for study, to figure out what had happened, and then as many times happened in the history of Shinigami Science, (hypocritically)unethical experimentation began.

Established Shinigami volunteered to wield the blade and we consumed. Students from the Academy tested it and were consumed. Random souls from the Rukon, volunteering just for a warm place to sleep that night or a good meal, tested it and were consumed. The sword was even presented to a handful of hollows, speculating that a hollow like Asauchi might react better to a hollow, only for the hollows to themselves be consumed. It was speculated that perhaps an Arrancar could handle it, but Arrancar are so rare in nature that testing that theory was deemed unfeasible and that no good could come from further study.

So, the sword was sealed away and hidden in the mortal world in the wilderness far away from what the Shinigami at the time considered civilization, as they saw it as potentially too valuable to destroy but feared what could happen if it fell into the wrong hands or worse, if those wrong hands somehow new how to use it properly: The Sword was composed of a lot of souls and devoured a lot more. If someone were to manage to properly bond with it not only would it become the ideal weapon to draw out that person's power, but it could potentially grant them much, much more.