You’ve been blessed with one of the greatest powers capable of being granted to Shinobi, and one of the Shinobi world’s greatest curses. Within you there had been sealed a great being of power, a beast made of incredibly powerful chakra has now found it’s home deep within your soul.
Yours is a bit of a unique case as the beast is no longer sealed within you, it instead has taken a liking to you and stays with you out of its own free will. It has bestowed upon you all it’s awesome power granting you everything from its chakra cloak, tailed beast bomb, its full transformation, and all its unique abilities. The beast can also freely leave to fight alongside you without any detriment to you, and you can easily command its power as you would your own freely mixing it with any other ability you possess.
The beast can be of any form you wish, whether a canon bijū or a unique one. Regardless of which beast you choose they are perfectly loyal to you and can not be taken from you without your consent.
The beast’s power will be on par with Kurama’s if his overall power would be at least 9 times your own and if you are more powerful the beast’s power will increase until it’s at least 9 times as powerful as you. Your strength can grow to match your beast’s, and if you rise past that point your beast’s power will rise alongside your own. Regardless of its strength in comparison to yours, combining your two powers will multiply your combined power by a factor of 9. If you choose a unique beast you may design a unique technique or ability it has such as Shukaku’s sand or Son Gokū's lava.