No one would expect a Great Mage to murder his own servant, especially one that came from Dalaran the Magic City.
From the Worgen's description, Lu Li could tell there was something that he wanted to avoid talking about. Lu Li suspected that he was most likely about to die from an illness or had suffered a serious injury.
The Great Mage Ur needed a soul and so naturally, he chose his beloved servant.
Sosa Modnaire - that was the Worgen's name when he was still a mercenary. His soul was transported to a Worgen's body by Ur's dark magic.
He wasn't a simple Worgen like the others; he was a creation of Ur's hard work and dedication to his passion.
Sosa Modnaire was sealed away in the book by a curse. Even the second owner of the Book of Ur did not even know about this. Arugal, in fear of the book being stolen, hid it away in Shadowfang Keep.
In terms of how this relates to the game, only monsters from Shadowfang Keep had a chance of dropping the Book of Ur.
