The Nine Tails

"be careful when choosing this subclass"

yeah because the dm will kick your ass OUT

This is worse than most of the lower rated homebrews

In order to help you improve this subclass/future homebrew:

1st Level) Limit abilities, that can prevent death. An example is Relentless Endurance from Half-Orcs: "When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead. You can't use this feature again until you finish a long rest." Also specify the amount of healing gained, if you mention it. Healing to full after a fight is broken, especially if you don't specify, what consistutes "after a fight". The round after no more enemies are on the map? After 1 minute? This could be made more balanced if for instance you got to expend hit dice say 1 minute after a combat, where expending 1 hit dice takes an action/bonus action.

6th Level) This is absolutely broken as well and needs to be changed.

10th Level) This is too much for a single level. Change it to be similar to the Tough Feat perhaps, which would be very lackluster but at least it's not broken then.

14th Level) What defines "on the field"? On the battle map your DM has made for you to play on? What if your DM likes to do fights through theater of mind? Is there anything that limits how far you can "rush" to the enemy? Attacks should either require a roll from you or a save from the enemy. Does using this feature require your reaction? Either way, since you rarely ever see a party member die, I guess the level 14 feature is actually the only one that is underwhelming. I would change it regardless, since you'll likely barely ever get to use it.

Hmm. This is balanced.

so you dont die and everything else does. good to know.

so.... immortal, can cast any spell ever for free and a shitton of hp. got it.....