The sixth book in the Diablo series and the first book in the "The Sin War" trilogy is called Birthright. Like all the books in this trilogy, Birthright is written by Richard A. Knaak.
Birthright tells the story of how Sanctuary was first created. The angel Inarius and the demon Lilith (child of Mephisto) grew tired of the endless conflict between the High Heavens and the Burning Hells, and wanted to create a safe place where demons and angels could live together. Using the power of the Worldstone, Inarius used the powers of the Worldstone to create the world known as Sanctuary, and also used the power of the Worldstone to keep it hidden from both demons and angels. Demons and Angels of similar mindset also joined them on this planet, and together they created the Nephalem, the first generation of humans. Inarius and Lilith also had a child of their own called Rathma.
Things did not keep going according to plan however, as Lilith decided to kill all the Angels and Demons (save for Inarius) and use the Naphalem to build her own army. Inarius grew angry when he learned of this, and killed many of the Nephalem. Inarius then altered the Worldstone to not only keep Sanctuary hidden, but also weaken the Nephalem over each generation, which is how the Naphalem turned into humans over time.
Lilith still wanted to create her weapon, so she found a human with potential (a farmer named Uldyssian ul-Diomed) and awakened the power inside him. In his time, Uldyssian gathered followers that also had power like his, who followed him unqestionably as their leader and teacher.
In a nutshell, Lilith's plan backfired, but you will have to read the book to find out what Uldyssian did.
While not the best work written by Knaak, this book is pretty good for Diablo fans. It has good tension in the storyline, good mystique, and generally good things that keep you interested. What I didn't like about this book is that in some parts of the book Knaak gave too much of the story away,and the fact that he keeps using long phrases such as "the brother of", "the son of", etc instead of the character name throughtout the book, as if just for filler.
For a book priced less than $10, you can't really go wrong here.
The book is available as a standalone book here:
Or as part of the "Sin War Archive" trilogy here: