666 was a number.
666 was known as "the number of the beast". (PROSE: Warlock [+]Loading...["Warlock (novel)"])
When the Midnight entity possessed Sky Silvestry, Jethro Cane said 666 to see if she would repeat it. (TV: Midnight [+]Loading...["Midnight (TV story)"])
The Tenth Doctor guessed 666 as a security protocol number for the Heavenly Host. (TV: Voyage of the Damned [+]Loading...["Voyage of the Damned (TV story)"])