Michael S. Heiser has been dubbed "Zechariah Sitchin's Worst Nightmare" by this website. The reason for that is, that he actually earned university titles with his extensive and profound reasreach into the ancient accounts of the Sumerians, Canaanites and Hebrews. To name a few, that is.
Michael Heiser therefore is an expert on ancient documents from the middle east. These documents all speak about the Watchers, who were fallen angels that made offspring with humans. This offspring was the reason why the Most High sent the Great Flood. We know the offspring under their name "nephilim".
In this lecture, he explains just who the Nephilim are.