Here are two fascinating stories partly in the Old Testament, partly in the Apocrypha (especially the Books of Enoch and the Book of Giants).
THE GRIGORI AND THE GIANTS
In the first story, God sent down a number of angels to watch over and instruct the children of Adam and Eve. Under the influence of their leader, the angel Samyaza, many went beyond their allotted tasks and secretly instructed humans in forbidden arts and technologies, e.g. weaponry, cosmetics, mirrors and sorcery. Even worse, they fell in love and lust with mortal women, engaged in sex and produced offspring!
The Watchers or Grigori who stayed true to their task are described in the Book of Daniel; the bad watchers come in for a mention in Genesis 6:1-4, where ‘Sons of God’ is generally interpreted as referring to the latter angels:
“When men began to multiply on earth and daughters were born to them, the sons of God saw how beautiful the daughters of man were, and so they took for their wives as many of them as they chose … At that time the Nephilim appeared on earth … after the sons of God had intercourse with the daughters of man, who bore them sons.”
The offspring are called the nephelim or gibborim, a race of hybrid giants who wreaked havoc on the earth to the point of endangering humankind. When God saw their depredations, he sent the Flood (among other reasons), and the giants were wiped out.
Meanwhile, the fallen Grigori were duly punished, suffering appropriate punishments for their lapse. The Hell or Gehenna to which they were sent is not the earlier Hell of Satan, but a secondary Hell within Heaven itself. Interpreters usually locate it in the Fifth Heaven (Fifth Altitude of Heaven) or sometimes in the northern regions of the Third (Altitude of) Heaven.
NIMROD AND THE TOWER OF BABEL
In a less common variant, the giants or nephelim help to build the Tower of Babel.
The story of the building of the Tower of Babel is in Genesis, 11:1-9. In sources outside the Old Testament, the builder of the tower was King Nimrod, a powerful and tyrannical ruler, and the act of building it was an act of hubris and disrespect towards God. The top of the tower reached almost to Heaven, and it took a full year to climb to the top.
The hubris takes different forms in different rabbinical texts. In one, the builders attributed the Flood to a weakness in the structure of Heaven and aimed to erect supporting columns to prevent another deluge. In another version, the builders sought to pierce the underside of Heaven in order to find out whether it was made of clay or brass or iron. Most extreme of all is the version where King Nimrod and his courtiers stood on a platform at the top and shot arrows up into Heaven – which came back dripping with the blood of angels! This led them to believe they could successfully wage war against God.
In all cases, God acted first. Whereas all people on earth had previously spoken the same language, He cursed the builders – and ultimately everyone everywhere – with different languages, so that they could no longer communicate with one another. Hence the multitude of human languages ever since; hence the modern usage of ‘Babel’, meaning confusion and chaos.