I have already written an spoiler afew days ago about his fate
It's vague
He will physically die here but his spirit remains and ancient demons previous demon kings use most of their power to break apiece of the world and connect him to it so his spirit becomes half immortal
In novel chapter 629 he descend and give final farewell to main character and formula of immortal wine and tells him you can give the formula to your disciple this is how you will keep my legacy and I live through thousands of years
And he says he can't meet the main character again since he disturb the balance of the world by showing himself
After that his spirit becomes motes of light and fly to sky/heaven
Some says he Ascended some say he destroyed his own spirit
In my opinion the motes flying to heaven means Ascension
BTW currently it's chapter 390 of the novel novel finishes at chapter 650
After that there is extra side story about demon lord daddy
From chapter 651 to chapter 1151
It's about his beginning when he joined the Demon cult to his Ascension to demod God position and after that will be 10 years later when main character joins the cult and chapter 1 of the main story
So in my opinion since author wrote a full story about him
Him being dead as the knowing ending is impossible for people to read the story no one likes the ending where main character is dead so he is alive and Ascended in my opinion
Di he die in the novel?