Ok this is a newish thing im introducing to the guild.
Hidden Arts are hidden 'Releases' that are not known by anyone but the Kage of your village or NPC's (teaching purposes).
The people in you village can only find out about it by witnessing the jutsus the Releases give. Even then you can only guess what it is till the person tells you >.> (of course if someone says 'Poison Release: Something something'.. then you know its a release of some sort but you don't know its a hidden art)
The Hidden Arts have strict rules and not everyone can get one.
There is only going to be a Total of 4 Hidden Arts.
1. Poison Release (only complete one right now)
2. Iron Release (IC)
3. Oil Release (IC)
4. Panthers' Release that i dont know the name of yet (IC)
You must follow the rules which are listed in the jutsu list for the release. Once obtaining knowledge of your hidden art (must apply for Hidden arts) You must travel to the hidden art's village and pick up your ranks scroll. Each village for the hidden art's has a training ground that give you bonuses to help learn the hidden arts. (These are not considered Ninjutsu or Genjutsu or a element so any bonuses that effect ninjutsu or genjutsu do not apply to the hidden arts) More details in the rules of each art.
I hope you enjoy them as i finish working on them >.>
Note: Poison Release is the only one that's going to be limited due to how it is set up.