You can read a good deal about the nephilim and their fallen angel parents from the apocrypha. Specifically, I Enoch.
Basically, the angels that did the nasty with humans, were assigned the task of watching and reporting on said humans. They were given the title of "watcher". The descriptions of the watchers are not terribly pretty, much like other "inner circle" angelic beings. Things like flaming wings, many animal like heads, et al.
These watchers decided that human women were damn sexy, and basically got horny, even though they did not have the parts to do the needful. Because they did not have said parts, they conspired together to change themselves, and made a solemn oath pact that if any one would do this, they all would do it-- and then having sworn that oath, they did. They changed their bodies from perfect divine beings into semi-corporeal ones, so that they could physically interact (ahem) with said women, and being angelic beings that had intimate knowledge of heaven, they shared that knowledge with their new wives and the children they produced. (sorceries)
Needless to say, this was outside the bounds that the divine creator had established, either for celestial beings, who did not need to procreate because they do not experience death-- and for mortal beings, who are not supposed to have powers or abilities that intersect with the divine. Due to this "out of scope" problem, the nephilim had corporeal needs that were not easily met (they needed lots of food), and resorted to cannibalism of ordinary humans after all the local sources of food were depleted. It was when humans started crying out about this cannibalism that the divine creator put his foot down, and did the great flood.
Let me see if I can find I Enoch for you. (II Enoch is filled with very dubious astronomical observations, and is not useful for this discussion.)
https://www.ccel.org/c/charles/otpseudepig/enoch/ENOCH_1.HTMThere we go.
If you are just interested in half-humans though, sumerian/assyrian mythology is far older than biblical, and prominently featured such beings. The epic of Gilgamesh is about this very thing, as the king of Uruk, the eponymous Gilgamesh, is such a being. (his mother was a goddess)
https://en.wikipedia.org/wiki/Epic_of_Gilgameshhttp://www.ancient.eu/gilgamesh/