They are the children of humans and angels: "the heroes of old, the men of renown."
Enoch 6.1-4 'And it came to pass when the children of men had multiplied that in those days were born unto them beautiful and comely daughters. And the angels, the children of heaven, saw and lusted after them, and said to one another: "Come let us choose ourselves wives from among the children of men and beget us children." ... And all the others together with them took unto themselves wives, and each chose for himself one, and they began to go unto them and to defile themselves with them, and they taught them charms and enchantments... and they became pregnant' - The Secret History of the World
So it begins, the timely fall of our heavenly beings, angels who lusted for a taste of the what the world can offer. As it was said in the Book of Enoch, there were angels who took in daughters of men as wives and produced offspring; these offspring came to be known as Nephilim: children of Heaven and Earth.
Who are they?
Nephilim directly translate to “the fallen ones:” hybrids, giants, demigods, celestial spirits trapped in a human body. The offspring of heavenly bodies and earthly women, these beings looked like humans and moved like humans, although they were slightly larger and taller than any ordinary human. They were also rumored to have wings and unnatural strength and power. But despite these unusual physical attributes, they had the grace and beauty of angels.
What is their purpose?
Their time on Earth was brief, according to what the accounts say, but not without stories and speculations. While they stayed here on Earth, there were stories of how they battled demons such as vampires, werewolves, and faeries. Allegedly, they did this to protect human beings.
Ezekiel 32:27 They do not lie with the fallen warriors of ancient times, who went down to Sheol with their weapons of war, having their swords placed under their heads and their shields on their bones, when the terror of these warriors was in the land of the living.
In other accounts, their name literally translates to “fallen ones,” and their prime purpose is to be with the fallen ones. They were there to bring destruction to the Lord's plan for the world. But aside from being the fallen ones, they are also known to be warriors who fought for what they’ve known is right. As the book of Ezekiel would say, they have lain with the warriors with their weapons for war.
Going back to the story of Noah and the great flood that washed away all of God’s creation on Earth and wiped out humanity. They said that it was God's way of cleansing the Earth of man's wickedness.
Genesis 6:5-7: The LORD saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time. The LORD regretted that he had made human beings on the earth, and his heart was deeply troubled. So the LORD said, “I will wipe from the face of the earth the human race I have created—and with them the animals, the birds and the creatures that move along the ground—for I regret that I have made them.
With the Lord's disappointment with man and how they tempted and corrupted his sons, he had decided to wipe all of them from existence, wiping out the Nephilim as well. It was said that they were made to fall, as their name indicates, in their biggest fall of all. Despite their unnatural powers and angel blood, God deemed them unworthy to exist. Yet, despite their deaths brought by the flood, it was said that their line continued in some of the known people in the Bible, fulfilling their purpose to become hindrances in God's plan and being among the fallen ones.