According to The Three Books of Enoch, a set of books that are non-canonical, Semyazza was a 'Watcher.' That is, he was part of a select group of angels picked by God to help the Archangels build Eden. The story goes that when the Watchers saw the daughters of man and became enchanted by them. They began spilling the secrets of man.
The story goes on that God was so angry with them he threw them out of Heaven and cast them to Earth as mortal demons, the Nephilim, where they created such a disturbance on Earth that God sent the flood to cleanse the planet.
Semyazza was a Grigori (meaning "Watchers" in Greek), a group of fallen "sons of God" who descended upon the earth to mate with human females.
The Grigori were bound to the earth during the Great Flood as punishment for their fall.
A single verse from the Book of Genesis in the Old Testament alludes to the "sons of God" intermarrying with the "daughters of Man" to create a species known as "Nephilim".