Unknown beast
After the fall of the bijuu and their destruction, many people attempted to create new bijuu by the use of jutsus, resurrections, and the undead. All of them had no effect on people and were complete failures... all but two.
The first one took place in what is now Kumogakure. The experiments by an unknown scientist were on the dead in which the scientist attempted to recreate the bodies, but break the limit on the human body. Soon after started, he realized that a massive chakra increasement would need a massive amount of bodies. As a result, he kidnapped many people and dug up graves. As a result he created something he called "Tairei" or great spirit. The Tairei separated into many small spirits but were ultimately controlled by a stoned called the "hitoride Gem" Or the alone gem.
Today, the Hitoride gem is in control by the presence of a group of people known as fushoo or Unidentified. Members of this group are rumored to never leave their mask or have the ability too. It is said that the mask symbolizes eternity among the group and can not be removed. It is also said that the mask symbolizes rank among the group as each is covered in unique symbols and designs.
The more markings the more power. However, ever new member has a blank mask.
The second invent to take place was preformed by a doctor by the name of Matsu Moshi, on himself. He felt that if filling his body with the remains of the bijuu, he could recreate it's power. He succeeded, but his body soon deformed the more and more he used it. He realized that by using a gem that he created he could control it, but he soon went mad and killed himself. However, the chakra of the bijuu took hold of his mind and recreated a body. This body has appears to be black and does not have a face, but is able to communicate through the mind.
It is said that by him touching you, you will gain a small out of bijuu powers. Those with these powers will have the ability to slightly transform their body ( wont become a 200 ft monster.. still normal size) and will have access to an array of bijuu like abilities at the cost of their bloodline. These people become known as yakan
How these two events tie together
If you recall, Matsu Moshi preformed experiments on himself, but died later on. His body was then recreated by the bijuu. But what about his old body?
Matsu Moshi filled his body with the remains of several of the old bijuu, and many bijuu in one human collided and separated. And that dead body soon walked again thanks to the bijuu or more specifically the 9 tails and traveled to Kumogakure after taking the gem that was going to be used to seal the bijuu before Moshi died.
The body of Moshi and the unkown scientist from Kumo that created the unknown are the same people.
So I bet you are wondering what happened to the bijuu that the 9 tails suppressed with the gem?
He, during the creation of the Tairei, sealed the remains of the other bijuu inside of the Tairei just before sealing it into the gem. He then used this gem to create a far more stable form of the yakan at the cost of the power.
But here is the catch, the Unidentified, being stable and lacking of power can kill a Yakan and absorb their bodies for power which brings more markings on their mask
The Tairei, being powerful have poor control over their powers and need to kill a Yakan to gain control. The more control they have, the more abilities they will get, until they are indistinguishable from a human.
[So where do humans come in?
Thanks to the combined efforts of the Mist village and the leaf village an opposite reaction to the Hitoride gem was created. This was called Kiboo(hope). This gem was then broken into a bunch of smaller gems.
Being the opposite of the Hitoride gem, Kiboo can supress the powers of both The Unidentified and the Yakan.
So where do the Seven Swordsmen come in?
The remaining members of the Seven Swordsmen are in control of ALL the Kiboo gems, but cant use them while their sword is active
I will explain more if you want