The reference is not in Isaiah that I can find right now about the fate of the off[spring of the Fallen Angels but here in Enoch Chapter 10.   

 

11 ’ And the Lord said unto Michael: ’Go, bind Semjaza 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 judgment and of their consummation, till the judgment 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.

14 And whosoever shall be condemned and destroyed will from thenceforth be bound together with them to the end of all generations.

15 And destroy all the spirits of the reprobate and the children of the Watchers, because they have wronged mankind.

16 Destroy all wrong from the face of the earth and let every evil work come to an end: and let the plant of righteousness and truth appear: and it shall prove a blessing; the works of righteousness and truth’ shall be planted in truth and joy for evermore.