Genesis 6 talks about he ‘sons of God’ taking wives among the ‘daughters of men’ and producing offspring. The story is also told in 1 Enoch which is another ancient Jewish writing. These offspring were 450 foot monsters who had large appetites and started eating men so God sent the Noah flood. The monsters were eventually thrown into a pit and what came out of their bodies are today’s demons.

(The Great Courses – How Jesus Became God)