[Gen 6:4]:

"The Nephilim were on the earth in those days - and also afterward - when the sons of God went to the daughters of men and had children by them. They were the heroes of old, man of renown"

From 'Babylon, the Eclipse of Eden', by Pastor Dan Hayden:

"...The term 'sons of God' is used of angelic beings (Job 1:6; 2:1) and, therefore, refers to fallen angels bearing children through demonically possessed women................

The offspring were obviously supra-normal, perhaps even superhuman (Gen. 6:4), demonstrating a disruption of normal genetic patterns. Also, there is an otherwise unexplainable phenomenon of a special group of evil spirits who committed a particularly heinous crime and are now 'reserved in everlasting chains under darkness unto the judgment of the great day'

[Compare Jude 6]:

"And the angels who did not keep their positions of authority but abandoned their own home--these he has kept in darkness, bound with everlasting chains for judgment on the great Day."

[2 Pet 2:4]:

"For if God did not spare angels when they sinned, but sent them to hell, putting them into gloomy dungeons to be held for judgment."

hell = lit. "tartarOSas" = Tartarus, not hell but a holding compartment in the center of the Earth, "to be held for judgment".

This event is linked by Jude to other scriptural accounts (unbelief after the exodus, and Sodom and Gomorrah) for which he was calling Christians to 'remembrance' (Jude 5). If the 'angelic' account is a scriptural event to be remembered, as he seems to indicate, there is no reference in the Bible other than Genesis 6, which could fulfill this scenario. The claim that angels could not produce children through humans (based upon the statement that angels in Heaven do not marry, Mk 12:25 doesn't necessarily mean that the demons of earth cannot [or could not in the past] have sexual relations with women.......................

It is also interesting to note that Peter links the 'angelic' incident, referred to above, with the flood of Noah (2 Pet. 2:4-5), and John tells us that in the Day of the Lord, at the sixth-trumpet judgment when Babylon comes into final remembrance before the wrath of God (Rev. 16:19), four incarcerated demonic angels are set free from their prison in 'the great river, Euphrates' for the purpose of slaying 'the third part of men' (Rev. 9:14-15).