It's a code for his real name.
We've all heard about the infamous number 666, which is rather cryptically referred to as the "number of the beast" in the New Testament, and has more recently become known as the number of the Antichrist.
But what is it about this simple, symmetrical number that's been giving everyone the heebie jeebies for the past 2,000 years? It's just a number, right? Or is it...?
As the boys from Numberphile explain, 666 doesn't really have any remarkable mathematical properties, but if you look back into its history, it reveals something pretty incredible about the way the Bible was originally written.

Turns out, what you see in the The Book of Revelation where the writer refers to 666 in an apocalyptic vision isn't entirely what you get.
Put simply, 666 is being used as a code, and not a particularly subtle one, if you were alive and literate at the time of the New Testament.
This text was originally written in ancient Greek, where numbers are written as letters, as they are in Hebrew - the other main language of the original Biblical texts.
For small numbers, the first letters of the Greek alphabet, alpha, beta, gamma, represent 1, 2, and 3.
Then like in Roman numerals, when you want to form big numbers like 100, 1,000, 1,000,000, they're represented by their own special combination of letters.
"Now what this means is that every word also has a numerical value," says Pete in the video above. Keep that in mind.
So back to the Bible, where in Chapter 13 of The Book of Revelation, it reads: "Let the one with understanding reckon the meaning of the number of the beast, for it is the number of a man. His number is 666."
As the video explains, the English word "reckon" comes from the Greek word for "calculate" or "solve". "So it's almost as if the text is saying, 'I'm gonna give you a riddle, you need to calculate the number of the Beast,'" says Pete.
So what does the number 666 mean when you translate it out using the Greek alphabet?
Well, given the hatred of the Roman Empire at the time, and particularly its leader, Nero Caesar, who was considered to be especially evil, many historians have been looking for references of this in the Biblical text, which was not written in a vaccuum, and was very much a product of its time.
When you actually look at the original text, you'll see that in this passage, the letters of 666 are actually written in Hebrew, which places a higher significance on numbers meaning words and words meaning numbers than ancient Greek. The writer was very clearly trying to tell us something.
And sure enough, if you translate the Hebrew spelling of 666, you actually spell out Neron Kesar - the Hebrew spelling of Nero Caesar.
Even if you take the alternative spelling of the number of the beast, which has been found in several early Biblical texts as being written 616, you can translate that out as being Nero Cesar.
"It adds to the kind of complexity of it kind of being a riddle, a secret," says Pete in the video above.
"No one wants to write a book under imperial persecutions saying, 'The root of all evil is Nero Caesar.' You're not going to spell that out."
Now, when I said 666 doesn't have any particularly remarkable mathematical properties, that's not entirely true, because it's actually what's known as a triangle number.
To find out what that means, and to understand more about the connection of the devil's number to the roulette table, watch the Numberphile video above.