Adopted Soldier

The Mutsu clan is one of the finding clans of Berdonia, they were a family of assassins that worked directly for the Emperor of the Granado Empire. When the Greyhounds convinced the two slave brothers that would later be known as the founder of Berdonia to revolt, the Greyhounds knew that the slaves even with their help wouldn't really be able to do much. So through some secret maneuverings here and there, the Greyhounds were able to get the Mutsus to help. 

Though the Mutsu clan were a clan of assassins, they still had their own morals that they upheld. Unfortunately for them, their master at the time was the Emperor of the Granado Empire, who back then didn't really care about any of those so-called morals. The old Emperor would usually order the Mutsu's to kill any noble family that was suspected of simply thinking about revolting. Some of those noble families were innocent, but that didn't matter to the Emperor, for as long as there was an inkling of suspicion they were erased. 

The Emperor even ordered them to kill innocent children that he feared would grow too powerful. The Mutsus didn't want to do all of these evil deeds, but they were bound to the Emperor, and they had nowhere else to go. When the Greyhounds gave them a way out, they gladly took it. 

With their knowledge of the ins and outs of the Empire and the positioning of all the soldiers, they were able to successfully escape with several thousands of slaves. As a final act of loyalty to their former master, they decided not to kill the Emperor even though they could've killed him anytime they wanted. 

From then on the Mutsus became free, but still, they kept to the darkness. They supported Berdonia in the darkness. They continued their business as an assassin family and took a coverup business which was creating masks and they were also doll makers. 

Now that they were free from anyone's control they started choosing jobs that would benefit the people. They only accepted targets that were deemed extremely evil. They continued to do the heartless job of killing for the greater good, for the next several hundred years. 

...

The Mutsu clan even after their separation from the Empire, were still and forever will remain as the greatest, the strongest assassin family to ever exist, and the reason for that is because of one being, Shadow. 

To the outside world, to those who knew the true face of the Mutsus, Shadow was the immortal leader of the Mutsu clan. He was the feared assassin that could never die and has never failed to kill his target, he was the perfect assassin 

Yet the truth was, Shadow, was simply the title given to all the heads of the Mutsu clan. When the old Shadow nears his death, he is replaced by the strongest of the younger generation. Then once the new Shadow is chosen, he is given the right to gain all of the true techniques of the Mutsu.

Back in the olden days, the head of the Mutsu clan, Shadow would write in a scroll, in that scroll was all of his experiences. In that scroll, he would write all the techniques he has ever created. He would also write down his own understanding on the art of assassination. When the next Shadow is chosen he is given the scroll and is then given a few years to learn all the abilities within it. 

When he becomes the Shadow, he too would add content to the scrolls, but during an emergency for when the former Shadow dies in the middle of a mission and fails to kill the target, the next Shadow is supposed to kill the target first before he's to learn the secrets within the scrolls. Of course, he is given some of the basic techniques of the Shadow, before he is sent to finish the mission of the previous one. This was done so that the people wouldn't think that Shadow actually died. Making the new Shadow finish the mission just adds to the myth of immortality. 

This was the system of the Mutsu clan for a few hundred years. The system changed in the age of technology, the Mutsus had incorporated the use of technology in the art of assassination. It was also because of this that the passing of knowledge to the new Shadow was made easier. 

The Mutsus created a device that stores memories. To whoever is chosen as the new Shadow, he or she can access the device. Once the device is used all the knowledge of all the previous Shadows are forcefully passed down to the new one. It is in that moment the former person that he or she was before becoming Shadow is supposedly erased. This is the reason why every subsequent Shadow is stronger than before. This also gave birth to another legend of Shadow, that his strength greatly increases every time he dies.

With this strength, the Mutsu clan became the literal monster under the bed.. This then gave birth to the saying, when the Shadow arrives only death follows.