The Knights Who Say Ni!are a band of knights from the comedy film Monty Python and the Holy Grail, feared for the manner in which they utter the word "ni" (pronounced /ni/, like knee but clipped short). They are the keepers of the sacred words: Ni, Peng, and Neee-Wom The Knights are led by a man who is approximately 12 feet tall with disproportionately short arms and reindeer antlers inserted into his helmet (played by Michael Palin standing on a ladder; the original screenplay suggested that he be played by "Mike standing on John's shoulders").
The other Knights are of normal human proportions and act as a chorus, only repeating words and phrases that the head Knight has spoken. "Ni!" is only the most notable of the sacred words which they are assigned to protect; the others being "Peng" and "Neee-wom". All of these words are infamous for the palpable horror and fear (and suggested pain) they bring about, whether delivered by the Knights or not.
According to King Arthur, "Those who hear them seldom live to tell the tale!"