Ninjas are a species of ghost bred for the purpose of assassination and other nefarious deeds. It is said that a ninja is able to move like the cat, glide like the owl, bark like the dog and sing like the moth. No-one ever hears a ninja's approach, because he stealthily places earmuffs on the watchmen and guards as he passes by.
A ninja carries deadly weapons of stealth, such as the death star - a huge planet which can shoot other planets with a laser - or the nun-chuck, which can chuck nuns. He dresses in black, because it is flattering to the figure, and covers his face with a mask to prevent freckling.
When a ninja strikes, it is always to kill. Occasionally the victim may escape with psychological damage and bodily wounding. These are referred to as 'ninjuries'.