Murai

Murai is a Master Ninja possessing unsurpassed skill in all the Ninja arts. He is Ryu Hayabusa's uncle from his mother's side. He was once a member of the Hayabusa Ninja Clan but events that occurred in the past caused Murai to leave and form his own rogue ninja organization, the Shadow Clan. His ninja fortress is located not far from the Hayabusa Village. 
 
In the events of Ninja Gaiden, Murai gets a visit from his nephew in his Shadow Clan fortress. Ryu tells Murai that his father Joe Hayabusa has entrusted him the Dragon Sword while he continues his training. Meanwhile, the sister sword, the Dark Dragon Blade, is being guarded by the Hayabusa Clan to ensure its dark powers is sealed. Murai expresses his feelings saying it is a shame for the Dark Dragon Blade's power to be unused but alas, since he is no longer part of the Dragon lineage, the sword is not of his concern. Soon afterwards, their discussion is interrupted by Ayane, Murai's pupil and apprentice. She brings urgent message that the Hayabusa Village is under attack by an unknown assailant and Ryu quickly responds to the call. The unknown raider kills Ryu's best friend Kureha, steals the Dark Dragon Blade and attacks Ryu, leaving him to faint in the rubbled village. The next day, Murai informs Ryu that the being responsible for the attack is Doku, Lord of The Greater Fiends of the Vigooran Empire. Ryu then sets on a journey to seek his vengeance.  
 
During Ryu Hayabusa's journey to seek for the Dark Dragon Blade, a mysterious being only known as the Dark Disciple has been following him in the shadows. He is neither on Ryu's side nor the Vigoor Empire's side. He remains behind the scenes during the events, monitoring Ryu's progress the whole way. The Dark Disciple is heavily interested on the Dark Dragon Blade and is hellbent in obtaining the cursed blade himself with the help of his apprentice, Gamov.  
 
At the end of the game, the Dark Disciple reveals himself as Murai and as he grabs the Dark Dragon Blade, he mutates into a fiendish form and attacks Ryu. Alas, even with the cursed sword, Murai is beaten, falls down into a bottomless pit and presumably dies. It is unknown if Ayane is aware of Murai's betrayal or is part of the evil scheme. 
  

Additional Info 

Murai is known to be one of the hardest boss to beat in Ninja Gaiden. He is the first and last boss in the game. His first boss fight is one of the hardest and his last boss fight is one of the easiest, which is strange being that it should be the other way around.