We know that the Rikudou Sennin after dividing the Juubi and creating the Bijuus give each of them a name:
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And that Naruto learned these names:
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Tobi/Obito doesn't know the Bijuus' True Names
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So by extension, Madara most probably doesn't know them either:
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I think there is a hidden power in the Bijuus' true names.
It is called Kotodama 言霊 or "Word Soul/ Word Spirit
It is an ancient Japanese belief that mystical powers reside in words or names.
One of the most prominent tale of the use of Kotodama concerns the Sun Goddess Amaterasu uttering Divine Word/ Kotodama and giving birth to deities:
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In Naruto, a prominent example of the use of Kotodama is found in the Treasured Tools of Rikudou Sennin particularly the Benihisago:
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"The Benihisago is able to take a person's word soul (言霊, kotodama) hostage by saying the verbal command "record" (録音しろ, rokuonshiro). After the word soul has been drawn out by the Kōkinjō and then severed from their body by the Shichiseiken, it is absorbed into the gourd. Completing these conditions "curses" the victim; if they utter the word they have said the most during their life, they will be sucked into and trapped within the Benihisago. Saying a homophone of their most frequently spoken word, or even saying nothing at all for a certain amount of time, will also trigger the gourd's suction."
In western religion and folklore the concept of Kotodama is called "True Names"
In Scandinavian Mythology, water spirits called Nix could be defeated by calling their names:
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In Kabbalism, the invocation of the true name of God was said to give the speaker power over creation:
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In Occultism, It is said that invoking the True Name of a Demon allows the speaker to summon and bind the demonic spirit.
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I think the Juubi's Final Form has a consciousness/personality, and much like Kurama and the Bijuus, it probably has a True Name itself (though not conferred by Rikudou rather a True Name it calls itself) unlike its' numerous titles/aliases:
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If the Juubi indeed has a true name, Kishimoto will most probably take inspiration from Japanese myths what to name it.
Two of the most probable names are:
1. Amatsu-Mikaboshi 天津甕星 The Primordial God of Darkness Evil in Japanese Mythology. Also known as the "August Star of Heaven"
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2. Sōjōbō
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It is said that the Ancient Ninjas of Japan takes their heritage from the Tengus, one such Tengu stood above all others, the God-King Tengu of Mount Kurama: Sojobo:
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The image of Sojobo in the manga was first seen in the Fire Temple:
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Sojobo was the King of the Tengus and resided in the forest of Mount Kurama, it is said that the legendary warrior Ushiwaka-maru was taught by Sojobo in the arts of swordsmanship, magic and mysticism.
I don't think Naruto, nor Kurama or the Bijuus know the true power of their names, it is more likely that Rikudou Sennin was the only one who knows how to use their Kotodama, so it's possible that the Bijuus' True Names will play a vital role in defeating the Juubi. It is most likely that Rikudou Sennin will teach Naruto the Kotodama when he appears inside Naruto's subconscious.
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