What was the nightmare of beginning. And a have a few more question.

Its been a while since i finished the novel and i cant remember everything well. What was The Nightmare of The Beginning? After the fight Jaehwan destroyed the world but in last chapter we saw Sirwen and others lived a human life till the old age. If the world destroyed how did they lived?

Also is there anyone who reads Revised Novel? I have a question about that, including twatf side story and more.