Muramasas Omusubi is an item in Ninja Gaiden 2 and it's remakes. When used, it restores all of the player's lost health and spiritual life. Ryu Hayabusa receives it from Muramasa himself in Chapter 10.
A package of Omusubi, or rice balls, hand-made by Muramasa himself. They are delicious.
Although this dish can be called either "Onigiri" or "Omusubi" in the Japanese language, it is important to note that Muramasa's products are definitely "Omusubi". To understand the difference, one must simply examine the etymology of the two words. While "Onigiri" is derived from a verb that means simply "to grasp or grip," "Omusubi" is derived from a verb that means "to bind or tie together". This is the key. A true "Omusubi" is not made only of rice, but also of the spiritual energy that binds the universe together. It is a symbol of the soul that struggles against the entropy attempting to unravel the world. As such, it is not only a source of balanced carbohydrates, but also of immense sacred power.