Angels and humans were never supposed to mix. But they did. And their children were not normal children. The book calls them the Nephilim. In plain terms: giants.
These weren't gentle giants. The book describes them as enormous, endlessly hungry, and violent. They ate through everything humans had. When the food ran out, they turned on people, and then on each other. The whole world filled up with fear and blood.
So picture the scene the book is painting: forbidden knowledge spreading everywhere, weapons in everyone's hands, and giant half-angel monsters roaming the earth. Humanity is drowning. The planet itself starts to groan under the weight of it.
That's a huge idea hiding in an old book. A lot of later thinking about demons and unclean spirits traces right back to this: they're the leftover ghosts of the giants, still wandering.