By nature, what makes a Nephilim is the fact that they have angels for fathers. The Bible and the Book of Enoch only ever mention sons, so it is unclear if any females were born.
According to The book of Enoch chapter 6, a group of angels called The Watchers made a pact on Mt Hermon concerning their schemes. They had seen that human women were beautiful , and they decided to have intercourse with them. This is also verified in Genesis 6:1, and 6:4. The result was the Heroes of Old, Men of Renown. While these titles indicate that the Nephilim were somehow valiant, the translation shows otherwise. In a later post I will describe in detail what their sons became.
The angels involved received just punishment for their actions; they became part of the Fallen, and were cast down to earth. To us.
Genesis 6:4 is clear that not only did this occur before the Flood, but again after. The question is when. At first glance, it appears that the Bible does not mention them again, until you dig deeper. This too will be discussed in the coming weeks.