Ryūjin Jakka (Zanpakutō spirit)

Zanpakutō Unknown Tales arc
When the rogue Zanpakutō spirit Muramasa appeared in an attempt to gain control of Ryūjin Jakka through his own Zanpakutō powers, Yamamoto acted first by producing a powerful barrier and going into a unconscious state so that Muramasa wasn't able to enter into his inner world to gain control over Ryūjin Jakka. Determined to achieve his goal, Muramasa had a contingency plan already in motion in case of such a set back. Muramasa staged a fake Zanpakutō rebellion by turning as many Zanpakutō spirits as he could loose on Soul Society.

Upon Muramasa's introduction to Ichigo Kurosaki, he devised a plan that would allow him to break the barrier Yamamoto had set up. By pretending Yamamoto was his prisoner, Muramsa was able to trick Ichigo into inadvertently breaking the barrier by combining his own power with that of Ichigo's Getsuga Tenshō at its most powerful (enhanced via his Bankai and hollow mask). Once the barrier was shattered, Muramasa freely took control of Ryūjin Jakka. Muramasa then used his new servant to incinerate Katen Kyōkotsu and Sōgyo no Kotowari in their attempt to kill Muramasa for using them. Muramasa then makes his escape, destroying the entire area in the process.

Powers & Abilities
Immense Spiritual Power: Ryujin Jakka is the oldest, most powerful fire type and has greater attack power than all other Zanpakutō in Soul Society. Its power is so great that even the strongest of Shinigami are afraid of it even at its base power.

Fire Manipulation: The release of this Zanpakutō comes with an extreme release of spiritual pressure affecting all in the area, and can be felt for miles upon miles across all of Seireitei. The aura of this spirit can disintegrate anything, changing it into nothing more than ash and engulfing the surrounding landscape in a blazing firestorm, the heat of which was intense enough to scorch the sky. Only powerful fighters such as Kyōraku and Ukitake can resist it, but even then only indirectly. Even in its Shikai, its power is enough to fight against the Shikai of two other captains simultaneously, both of whom had the reputation of being the strongest Captains in Soul Society while together. The flames created by Ryūjin Jakka can be controlled with great precision to attack only the targets it chooses and it also has power over the intensity of the flames. While Ryūjin Jakka physical manifested form has yet to be seen, it is able to itself become a giant raging flame.