Ancient Secret Technique: Bijū Synergization

An ancient technique that only Rikudō Sennin ever fully mastered in all his years of seclusion, and belongs solely to those who have mastered the power of a Bijū. It is special type of Jinchūriki Transformation that is said to be the purest and most-powerful kind, which Rikudō Sennin calls the "highest echelon of power." Also known as "the cold-blooded will of fire" by ancient Konoha scrolls, certain powerful jinchūriki are able to generate and manipulate this transformation. Mentally, it involves a complete absence of emotion or peace of mind, and physically it requires separating the energies of yin and yang. When the forces collide, the user only guides, rather than controls, the energy's direction. Because of this complexity, this attack takes much longer to initiate than standard attacks when first acquired. It also takes at least 7 years of training to grasp the concepts of it. It's powers can vary depending on how well it's mastered.

Rikudō, synergizing with the Ten Tails, was able to form this technique to be set apart by other means of bijū control. Seireitou, however, had obtained his own unique form of it. Changes to his hair color, gaining of kitsune ears, and overall tenacity of his chakra and physical appearance are all but unknown.