This post is a sequel to this post:
What would the original audience make of Gen 6:1-4? Some scholars think it alludes to heathen myths about gods coming down from the sky to mate with women and produce demigods.
In some Jewish apocrypha, it alludes to the fall of angels. I don't put too much stock in that interpretation because I have no reason to think Intertestamental Jews had the inside track on the meaning of Genesis.
I think it refers to people like legendary warriors, conquers, and founders of empires, dynasties, and city-states. Nimrod is a case in point (Gen 10:8-11). We don't have enough information to nail down who he was. Proposed identifications include Hammurabi and Sargon of Akkad. But that's a scholarly guess.
Another example is Menes, founder of Egypt, who reputedly united upper and lower Egypt. Whether he's the same man as Narmer, or a composite, is debated.
Gilgamesh is another notable example. Stories about their fabled exploits may well have been in circulation when Genesis was written. The text might have triggered those associations for the original audience. At the same time, the text takes them down a peg. They are mere mortals.
Someone might object that Gilgamesh is a counterproductive example. He was a demigod, right? But we need to distinguish between the historical Gilgamesh and the literary Gilgamesh of popular folklore or official mythology.
Moreover, even the famous epic has some plausible elements. It depicts his interest in monumental architecture and fortifications. He's portrayed as deflowering virgins. And it's quite realistic that a man in his position would, indeed, practice the droit du seigneur.