The Nephilim were fallen angels that mated with woman and bore giants before the flood,they were evil and caused havoc,The Nephilim giants spread violence and sin.God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was evil Thus God judged the Earth with the flood.saving Noah and his family.
Lots of theories, some fairly psycotic and/or imaginative, but no one truely knows since the Bible says almost nothing about them beyond a few verses. Something that doesnt exist on earth now, tho and made God very displeased.
The Nephilim are the offspring of the unholy angels. The angels are awaiting judgement day in Tartarus. Their offspring have not been bound, they are evil spirits. read Enoch. In the days of Nephilim, these half casts were massive, 300 cubits, if my calculations are correct, that's some 16 ft tall. They demanded too much off mankind. Once food shortages hit, they started devouring animals & humans. (meat wasn't consumed before this). They were sentenced to kill one another will the sword. These are the Greek god mythology fables, their stories originate from the Nephilim.
In the bible it refers to the "sons of God" mating with "daughters of men". Some take the to mean angels and humans mating, but some believe this refers to the two lines (cain and seth, evil and righteous) intermingling. The later is what i personally believe.