As far as I remember, the main character is confronted by other emperors, but loses due to his disciple betraying him because of some book, and he ends up killing himself, but is born in someone else's body. He ends up in a body of a small family's caretaker, and the story goes on from there.
Zhuo Yi Fan was called the Demonic Emperor because he became the strongest by using the Ancient Devil King’s Book of the Nine Secrets. Targeted by all the Holy Land experts and even betrayed by his own disciple out of greed for his powers he died. Then his soul enters and comes back to life in a family's servant boy named Zhuo Fan. Restricted by the body's feelings, he cannot ignore the family and the mistress he serves. Uniting the child’s memories with those of his former life, he will now lead this declining family back to the pinnacle of this continent and reclaim his seat at the top!
Haven't had time to read it yet, but based on the synopsis, it seems like a strong match to your description:
- MC starts out as a demon emperor
- Uses a magic book
- Is betrayed by disciple
- Reincarnates as a servant boy
I'll read through some of it and update this answer shortly, but I'll be surprised if this isn't it.