Muramasa was a nasty dog appearing only in the Shin Gaiden story.
Muramasa treats Shin, the real but young leader, as his slave. Shin is very afraid of Muramasa and obeys him. Muramasa has Shin's mother as his prisoner and threatens her life if someone from the pack tries to disobey him.
Gin comes with Ōu dogs to the area and after hearing what happened, they help the local dogs to get rid of Muramasa. Muramasa orders Shin to kill Gin but Shin eventually finds new courage and attack Muramasa instead.
In the end, it is Akame who defeats Muramasa by cutting deep wound on his neck.
"Curse you Ōu mutts! You'll never take these lands from me!"
- Muramasa's final words. Shin Gaiden
See more pictures in Muramasa/Gallery.