I strongly recommend that you download this, so you can see all the details!
After a painful transformation, the Rinji you knew was gone. Out of the fires of his aura arose a beast of unfathomable power and an uncontrollable rage. The worst case scenario has come to pass as the only remaining identifying feature left that indicates that this was once a controlled panther is the scar over his eye.
And while his Sharingan is unusuable in this form, it hardly matters when you can generate enough fire and heat to easily duplicate a nuclear explosion. Unlike the tailed beasts, the phoenix spirit is Rinji's own spirit, not a separate entity. Likewise he is still in control of his own actions.
However, like the other tailed beasts, the phoenix spirit is triggered by rage. And to be consume by such rage is almost as good as being controlled by another entity. There is only one way to subdue Rinji now, and the key lies in the pink crystal he wears around his neck. It's not longer visible, due to the extremely thick chakra form of the phoenix encompassing Rinji's original body.
It is possible for Rinji to transform into his phoenix form at will, however the sheer power he wields is still a very dangerous weapon that can lead to the destruction of the very planet he is on.
The only currently known instance where Rinji transforming into the phoenix was in his battle against Launa to determine who is the absolute most powerful. The planet they took their fight to was subsequently destroyed in the process from their combined attacks...
I want to personally thank ~egardn4
for doing this series! I am very happy with the way it turned out, and I can now finally have a visual frame of reference for a part of Rinji that has never been seen until now. Thank you!