Think,

“… None of such walked the earth again, the nephilims or the fallen sons of God who tried to make themselves as ‘a god’…”

They perished and are held in “everlasting chains” till the Great Day of their sentencing too:

“And the angels which kept not their first estate, but left their own habitation, He hath reserved in everlasting chains under darkness unto the judgment of the great day.” (Jude 1:6)

Their main sin & their destruction is described below:

“that the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose.

Then the LORD said, “My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years.”

The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men (heroes) who were of old, men of renown (legends, in many cultures) .

Then the LORD saw that the wickedness of man was great on the earth, and that every intent of the thoughts of his heart was only evil continually.

The LORD was sorry that He had made man on the earth, and He was grieved in His heart.

The LORD said, “I will blot out man whom I have created from the face of the land, from man to animals to creeping things and to birds of the sky; for I am sorry that I have made them.” But Noah found favor in the eyes of the LORD.” (Genesis 6:2 – 8)

P/S:

Only Christ the Lord is able to undo the “everlasting chains” by “His Mercy” in “His Will”. He decides… (mysteries, secrets).