Shinseitai

The Shinseitai (神声帯, Literally meaning: God's Vocal Cord) is a kekkei genkai of the Hatsune Clan, inherited from their ancient ancestor Kizune Saishone that gives the wielder a very unique larynx. Its appearance is very scarce in these days as most of the descendant has forgotten the joy of music, the main training method to master it.

Background
Its origin traced back to Ancient Era, where Ōtsutsuki first arrived. It was first activated by the ancient ancestor of Hatsune Clan, Kizune Saishone. It was one of the most well-known, feared and strongest kekkei genkai along with Byakugan of the Ōtsutsuki until Saishone Clan was annihilated by Kaguya Ōtsutsuki who had consumed the fruit of Shinju.

Abilities
Apart from having all the abilities of the normal human larynx, the Shinseitai gives the user the ability to manipulate their larynx in a very complex way. The user can change the size, length and shape of some part of the larynx including vocal cord drastically to generate voice at extremely different frequency, amplitude, and waveform than the owner's original voice. In layman's term, it means that the user is granted wider vocal range and ability to mimic others' voice.

The vocal range increased by the kekkei genkai varies between person. Some may only has tiny expansion of vocal range while some may experience ten fold of expansion. Although it depends mostly on the wielder's genetic, it can be improved through vigorous training.

At first glance, the ability seems to be pretty underwhelming for a kekkei genkai which was claimed to be the main factor for Kizune to dominate half of the world but in the Ancient Era where Chakra is not founded yet, Shinseitai's abilities was in fact very powerful. The ability of creating distraction by mimicking other people and animal voice or generate extremely powerful vibration allowed Kizune to become one of the strongest man.

After Chakra was shared by Hagoromo Ōtsutsuki, the descendants of Saishone Clan who changed their name to Hatsune found a new way to use Shinseitai offensively. By humming specific tune with Yin Chakra, they can hypnotize anyone who heard the tune. This ability, also known as sound-based Genjutsu, is rarely seen because it is very hard to learn as the specific tune is nearly impossible to be hummed by normal people and is difficult even for wielder of Shinseitai.

Trivia

 * Hatsune Clan was the clan who invented Demonic Flute. Its purpose is to assist the clan mates who are not able or having difficulty in humming the hypnotizing tune.
 * Demonic Flute grants the owner the ability to play some basic hypnotizing tune even if they are not able to hum it.