Rasetsu

Rasetsu is a human and Mazoku hybrid, being a primordial, 10 tailed mazoku of extreme power, holding A-class power within him. Rasetsu is a master of Youki and uses it exclusively, as his Youki is noted to be more powerful than most users of Seikoki or other mediums of energy. Rasetsu achieved the atavism of the Mazoku, transforming himself into a fullblooded demon and granting him an exponential increase in power.

Transformation
10 Tailed Beast State: Unleashing his latent youki, Rasetsu transforms into a titanic being of immense destructive capacity and insane durability. It is unclear if Rasetsu's mind actually dominates the new body that he obtains, as during the transformation, his entire body from head to toe becomes overwhelmed by an extremely dark black miasma which grows to the size of several mountains wide and tall, then solidifying into a huge, terrifying cyclops-like beast with ten tails hanging behind it.