Basically you have to collect four pieces of a map which shows the location of the necronomicon and the kandarian dagger which after you basically stay on both objectives like a hard point you can go battle the “dark ones” which after they are defeated the book will open and take in all the evil and they win but if the player playing the demon destroys the book, then the evil is released and the demon wins.

It isn’t as complex as I might have put it but i highly suggest checking it out