Marion, if you followed the link I posted earlier, you saw that the "old proverb" is quoted by J. J. Fux in 1725: mi contra fa diabolus est in musica, that is in English, "mi against fa is the devil in music". This is the earliest reference to an interval called "diabolus" (and doesn't even mention the tritone explicitly) that Ms. Shulter was able to find at the time she wrote that note, and it is pretty late. The reference may not be to any demonic beings. Singers who found the interval hard to sing may simply have called it, jokingly, and "interval from hell". The "diabolus" may have no more significance than that.

If you read through this note by Ms. Shulter carefully, you'll find that, if you tune around a circle of Pythagorean fifths far enough, you find that the "circle" never closes, but at some point creates an interval called a "wolf fifth". Ms. Shulter wasn't able to find how far back the "wolf" designation went, either. (Our modern 12-equal scale is designed so that the circle of fifths will close exactly.)

T.