So integral to metal, Slayer made a whole album in tribute under its other name: the Diabolus In Musica ("the devil in music").
Diabolus In Musica is a musical interval that spans three whole tones and pops up frequently in heavy metal; most famously in Black Sabbath's self-titled song. Metallica's Enter Sandman and Diamond Head's Am I Evil are other examples.
Away from metal, classical composer Gustav Holst used it to convey the power of Mars, the Bringer Of War in his The Planets suite.
Such are the tritone's devilish connotations, the church banned its use in the middle ages.
T is also for This Is Spinal Tap, because nothing has ever lambasted metal so accurately and amusingly as the 1984 mockumentary.
Is this where we insert the Tap quote?