In the Book of Enoch he is portrayed as the leader of a band of angels called the grigori that are consumed with lust for mortal women and become Fallen Angels.
"3. And Semjāzā, who was their leader, said unto them: 'I fear ye will not indeed agree to do this deed, and I alone shall have to pay the penalty of a great sin. 4. And they all answered him and said: 'Let us all swear an oath, and all bind ourselves by mutual imprecations not to abandon this plan but to do this thing.' 5. Then sware they all together and bound themselves by mutual imprecations upon it." - Chapter 6, Verse 3-5."
A full listing of the leaders of the group can be found on the 1 Enoch page. Semjāzā convinced several other Grigori to join him in fornicating with women. As a result, he and the other sinful Grigori begot giant offspring (in Genesis called Nephilim or 'fallen ones' in Hebrew) that dominated and feasted upon humans during the days of Enoch. The Watchers' other sin was to teach humans various creative arts especially Azāzźl's, who taught the secrets of war, which brought down the wrath of God.
God commanded the angel Gabriel to cause the Giants to wage Civil War:
"And to Gabriel said the Lord: ' Proceed against the biters and the reprobates, and against the children of fornication: and destroy [the children of fornication and] the children of the Watchers from amongst men [and cause them to go forth]: send them one against the other that they may destroy each other in battle: for length of days shall they not have." - Chapter 10, Verse 9
The judgement of the associates of Shemyazaz is described in Chapter 10 verse 12-14 of the Book of Enoch.
"11. And the Lord said unto Michael: 'Go, bind Semjāzā and his associates who have united themselves with women so as to have defiled themselves with them in all their uncleanness. 12. And when their sons have slain one another, and they have seen the destruction of their beloved ones, bind them fast for seventy generations in the valleys of the earth, till the day of their judgement and of their consummation, till the judgement that is for ever and ever is consummated. 13. In those days they shall be led off to the abyss of fire: 〈and〉 to the torment and the prison in which they shall be confined for ever. And whosoever shall be condemned and destroyed will from thenceforth be bound together with them to the end of all generations." - Chapter 10, Verses 11-14
After the destruction of the Giants, God caused the Great Flood (Noah's flood) to wipe out the humans who had become corrupted.
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