Devils, like other fiends, are considered to be children of Khyber. According to legend, following the titanic struggle between the three progenitor wyrms, the dragon Khyber, bound in the depths of the world, spat forth the fiends and demons, who ruled the world for millions of years.
Around 1.5 million years ago, with the help of the couatls, the dragons rose from their primitive state and began to revolt against their fiendish overlords. This war raged for over a million years.
Around 100,000 years ago, the couatls sacrificed most of their race to bind the most powerful of the fiendish Overlords back in Khyber, to be trapped for eternity within the Silver Flame.