666 is the occult “number of the beast,” also called the “sign of the devil” (Wang 1994), associated in the Bible with the Antichrist. It has figured in many numerological studies. It is mentioned in Revelation 13:18: “Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is 666.” The origin of this number is not entirely clear, although it may be as simple as the number containing the concatenation of one symbol of each type (excluding M==1000) in Roman numerals: DCLXVI==666 (Wells 1986). —Beast Number (Mathworld | Wolfram Research)