Maneseru no Oni

Maneseru no Oni was a particular Oni, appeared as a large, amorphous blob of a foul, black substance.

Maneseru lied in hiding within the Shadowlands, waiting for unwary samurai to come close. It then spawned duplicates of its foes and killed as many as possible, absorbing the dead, and slowly adding to its own mass. Creature of Rokugan p. 41