The satanic comma is the difference between 666 perfect fifths (octave-reduced) and a single perfect fifth. Equivalently, it's the difference between 665 perfect fifths (octave-reduced) and the unison -- but that wouldn't be as devilishly intriguing.
This difference is inaudible, at only 0.076 cents. The monzo is |-1054 665>.
It is tempered out in 665 EDO and its multiples (1330 EDO, etc.).