Hiraikotsu

is a unique gigantic boomerang and signature weapon of the demon slayer Sango.

Overview
Like the rest of the arms and armor of the people of the Yōkai taijiya village, Hiraikotsu was made from parts of vanquished yōkai. In particular, it was made from purified yōkai bones. Because of its supernatural material, it's nearly unbreakable and a very powerful weapon. It appears to be stronger than steel, while having a tremendous weight. Because of its quality, it requires a high quality weapon to cut it, with Tōkijin being the only sword ever to split it in half. Its latest repair by Yakurōdokusen after it had been destroyed, imbues it with an ability to destroy yōki like Tessaiga's Backlash Wave.

Sango uses Hiraikotsu in many different ways. The leather bands at either end allow her to grasp and handle it easily. At six feet in length and what must be fifty pounds or so, she handles Hiraikotsu very nimbly. After hefting it with one arm, Miroku comments that she must be very strong to be able to lift and carry it. Other than using it as a boomerang, she can swing it and wield it as a melee weapon. She's killed countless yōkai in this way, as well as hitting Miroku over the head when he was acting as a pervert around women. Hiraikotsu is also big enough for Sango to use it as a shield when the need arises, and the nature of its material makes it excellent for that purpose. When not in use, she uses a strap, possibly made of leather, anchored on either end by a buckle and using friction to hold the strap ends in place. She slings the strap over her right shoulder and the strap crosses down her body to below her left hip.

Sango has the basic knowledge and experience to repair Hiraikotsu; she likely was required to learn how to do so, as she sometimes went on exterminations alone. In the anime she had to return to her village to repair a large crack in the surface. The yōkai bones are worked as if the material was steel. A blacksmith's furnace is used to heat the weapon and bones until red-hot, and more bone is used to fill the crack, repeatedly struck with a blacksmith's hammer. According to Sango's comments, it was likely her father that taught her how to do so. While she admits that her father's handiwork would have been better, she was able to perform repairs of a quality that she was more than happy with.

Although Sango is very attached to Hiraikotsu, she is not beyond sacrificing it to save those she loves. While trapped by a dangerous Bone Demon, Sango destroys her valued weapon with poison salve to save Miroku and herself from being devoured. Sango and her comrades are led by Myōga to a mysterious yōkai known as Yakurōdokusen, whom Myōga claims can revive Hiraikotsu.

Yakurōdokusen agrees to fix it, dissolving Hiraikotsu in one jug and telling Sango to slay the yōkai in the second jug, warning her if she failed Hiraikotsu would dissolve into nothing. Sango then is determined to reclaim her weapon and jumps into the jug, and is quickly confronted by multiple yōkai. While trying to fight them, the yōkai angrily demand why Sango sacrificed them. Sango realizes the yōkai she's facing are what created Hiraikotsu. Hiraikotsu is furious Sango would sacrifice it for a human man, Miroku, and demands that Sango never betray it again and abandon Miroku. Sango refuses, declaring if Miroku died she wouldn't be able to live, thus, she would sacrifice Hiraikotsu again to save him. Hiraikotsu is further enraged and decides to kill Sango, however, it senses Miroku's resolve to protect Sango at the cost of his life and changes its mind. Hiraikotsu then admits it cannot understand human love but Miroku's courage has impressed it so it revives as Hiraikotsu to fight by Sango once more.

After Hiraikotsu is reforged, it gains the power to not only emit a demonic aura, but to also absorb miasma/poison and disperse it, making it much more dangerous against Naraku and enemies. Three years and six months after Naraku's death, she used it to fight against the Ne no Kubi.

Hanyō no Yashahime
Eighteen years later, the weapon is passed down to Hisui, Sango and Miroku's son. However, it seems to be normal as it lacks the ability to demolish demonic energies. When Inuyasha and Kagome return, Sango explains to her identical twin daughters that the time finally came for her to make use of the "Black Hiraikotsu" which she had been secretly forging and further strengthening with the help of Kin'u's ever developing spiritual abilities and Gyokuto's innate expertise in craftsmanship. This means that its abilities are considerably stronger and more effective towards their targets as they are deadly. She is seen carrying it on her back as she fetched her husband from his thousand-day spiritual training period.

A horde of yōkai later swarm toward Sango, as she is ready to display its unmatched demonic abilities to destroy them.

Powers & Abilities

 * Targeting Accuracy & Homing Effect: It automatically follows multiple targets until it hits and slices them to bits and returns to its owner's hand.
 * Yōki & Miasma Dispersion: Sango was forced to sacrifice the Hiraikotsu to the Bone Demon. After getting the weapon fixed through a trial involving the Yakurōdokusen's jars and the dozens strong yōkai spirits merged to form Hiraikotsu, the weapon was fixed as well as upgraded/modified greatly. Due to that, Hiraikotsu now emits a demonic aura, allowing it to inflict even greater damage to enemies. According to Naraku, it contains a "different poison" beyond his ability to inflict powerfully potent miasma, as he was unable to recover once he was struck with it. It can also absorb miasma/poison and disperse it safely.
 * Purification: Having been infused with Kin'u's Buddhist/spiritual power, the Black Hiraikotsu is capable pacifying multiple demons or evil spirits all at once, just like Moroha's longbow.

Name

 * Hiraikotsu's name is made up of 3 kanji (飛来骨). A kanji is a form of Japanese writing that each mean a different word by themselves, and can be combined to make new words; the first Kanji, pronounced with "Hi" in this case, means "Flight" or "Flying." The second Kanji, pronounced "Rai" in this case, means "to come" or "to return." Considering it's basically a giant Boomerang, the "Hirai" part is a way of saying in pure Japanese that it is a weapon that flies back to you. Finally, the last Kanji, pronounced "Kotsu" in this case, means "Bone," referring to that it's made from the bones of yōkai, and thus is basically one big bone. The name itself has been interpreted many different ways, but considering it's kanji and function, the term "Flying Return Bone," or possibly "Bone-a-rang," is likely the most correct translation.

Trivia

 * As stated by Miroku in InuYasha the Movie: Affections Touching Across Time, the Hiraikotsu is much heavier than it appears. Sango is able to wield it with ease, making it appear as though it's lighter.
 * It is possible that the Hiraikotsu could be based on Ukyo's giant spatula from Ranma ½ which is another series made by Rumiko Takahashi. Not to mention that both users of each weapon are voiced by Kelly Sheridan in the English dub.
 * In episode 44 of the anime, Kaijinbō cut Hiraikotsu in half while under the possession of the newly created Tōkijin, but it became intact in subsequent episodes without explanation. Tōtōsai was shown repairing it in chapter 163 of the manga, but not in the chapter's corresponding episode.
 * In episode 12 of InuYasha The Final Act, the yōkai souls of Hiraikotsu were voiced by, , and in the English dub.
 * Hiraikotsu's name resembles a weapon called Hikokotsu, which was used by a legendary demon slayer in Japanese Taijiya history 200 years ago.
 * A video was included in InuYasha the Movie 2: The Castle Beyond the Looking Glass that showed the feasibility of Hiraikotsu existing in real life.