Well, generally, it might be about a character who returns from the immortal world. Maybe they have gained special powers or knowledge there and now have to face new challenges in the mortal world, like dealing with old enemies or making use of their new - found abilities in unexpected situations.
The main plot could center around the protagonist's journey back from the immortal world. They might be on a quest to save the mortal world using the skills and insights they acquired in the immortal realm. It could also involve reuniting with old friends or family and trying to fit back into a world that has changed since they left.
In 'back from the immortal world novel', the plot may follow the character who has come back from the immortal world. This character could be trying to find their place again. There might be a power struggle as others in the mortal world either fear or desire the powers the character has brought back. The story might also explore the character's internal struggle of adapting to a less - divine environment after experiencing the immortals' way of life.