Ryūjin Jakka (Zanpakutō spirit)

is the manifested spirit of Genryūsai Shigekuni Yamamoto's Zanpakutō that is seen during the Zanpakutō Unknown Tales arc.

Synopsis
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, Muramasa 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 roof of the cave in the process. With Muramasa gone, Ryūjin Jakka prevents the captains and Ichigo from pursuing him. They make repeated attempts to break through his flames, but to no avail. Eventually, Tōshirō Hitsugaya and Hyōrinmaru arrive and freeze Ryūjin Jakka's flames with their Bankai, allowing Ichigo to escape. The others remain inside the flames. Yamamoto decides that it is time to punish his rebellious Zanpakutō.

Powers & Abilities
Immense Spiritual Power: Ryūjin Jakka is the oldest, most powerful fire type and has greater attack power than all other Zanpakutō in Soul Society.

Fire Manipulation: The flames created by Ryūjin Jakka are powerful enough to overwhelm Katen Kyōkotsu and Sōgyo no Kotowari and 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.