This chapter is not essential, but it can provide a better understanding of the story.
Enlil knows he's in the Douluo world because, in his previous life in Konoha Village, he bought and enjoyed a novel that helped him escape the burdens of war. The person selling these novels, including stories like Douluo and One Piece, was a reincarnated individual. These stories fueled Enlil's desire to travel to other worlds and have new adventures.(Enlil does not know that he is incarnate, but he thinks that he is a person of wide imagination)
When Enlil was on the brink of death, his deepest desire manifested as an unexpected ability: the power to travel to another world. The Mangekyo Sharingan's abilities, which reflect the user's soul and desires, made this possible for Enlil.
In the coming chapters, you will learn more about the reincarnated character. Is he truly from another world? What did he do in Naruto's world during Enlil's absence? Did he end the war, or was he preoccupied with other pursuits? All these questions will be answered soon, so stay tuned.