I LOVED that game, and since I played it about a year ago I have been trying to formulate in my head if it is possible to balance her book as a very special item, which can be the bases of a new campaign for my world/setting.

If I remember how it works, in the video game you basically first had to get an "Overkill", which basically meant first weaken the monster to near dead, then take it to around negative double it's normal HP with a single attack. I believe this represented "binding" the demon (hence the chains).

Afterward the book's owner could spend an entire turn (aka a full-round action?) to bind the demon to her book on a page.

The book has X number of pages, and after the battle a demon summoned from it to fight for her would go back to the book, or if it was killed the page would be open for someone/something new.

Any ideas? It almost seems as a class/prestige class itself, but I want it to be a special item that may see repeated uses in my campaigns. Also, some AWESOME fluff for it would be nice as to why it exist.

Also, should it only affect demons and/or devils? Or any outsiders? (How about Far-Realm entities. O.O)