What is it meant by singing the devils book?

Answer 1

signing the devil's book is to join the legion under Satan's command. It might be considered selling your soul to the devil, however there is no direct implication of the agreement in the phrase "signing the devil's book".

Answer 2

"Singing the devil's book" is an idiom that typically means someone is behaving in a wicked or immoral way in order to achieve their goals. It implies that the person is aligning themselves with negative or evil forces in order to succeed.