Demonicus de Zoltun Kulle
The Demonicus de Zoltun Kulle is a Vizjerei tome of demonic knowledge, primarily listing information about different demons. It's unknown how many copies exist of this book, Abd al-Hazir is in possession of one known copy.
The book is likely full of incorrect information, written by the Vizjerei as self-serving propaganda aimed at whitewashing heinous deeds committed by the mage clan during the Mage Clan Wars. Abd himself mentions the Khazra, and they are likely written about in Demonicus de Zoltun Kulle, but their true history would be covered up as lieutenants of Baal.
Contents
Volume 5
Only a section of Volume 5 is known so far:
Fallen Ones (Nanus Improbus)
As hard as it is to believe, the fallen ones were once exalted demons of the Burning Hells. They served as Azmodan's hands, performing acts that he would not, could not involve himself in. They were the instrument of Azmodan's first failed attempt to usurp power from Diablo and his brothers, and after that failure, the fallen were subjected to the full wrath of Diablo. They were twisted into small, ridiculous imps, in contrast to their previously powerful forms. Moreover, if they expected Azmodan to reverse their condition, they were sadly mistaken. The infuriated Azmodan held them responsible for the Prime Evils' continued reign, and so he left the fallen in their new bodies, where their degradation would serve to amuse him for all eternity. Their failure provided him with the information he required to succeed in dethroning Diablo and his brothers - the event now known as the "Dark Exile" - but that fact did nothing to soften his heart toward the fallen.
When unleashed upon our realm by their master, Azmodan, these impish terrors display a tendency to swarm like flesh-hungry locusts, and they have been known to tear apart a sleeping village in minutes. Small of stature and simian in appearance, these creatures possess surprising strength and unnatural agility. Other than feasting on human flesh, the only act that gives these unspeakable horrors pleasure is breeding; hence the tendency to encounter them in large packs.
However, due to their small size, cowardice appears to be one of the chief features of this species. They quickly retreat when one of their brethren falls in battle.
Fallen ones display no obvious tendency towards greater organization beyond their predisposition to swarm. This is fortunate for humanity, for they are so numerous that if they were to band together in large numbers, they might easily overwhelm a small city.
Trivia
Check out the Diablo I Manual for other similar in-world writings. There you can also read the "cover-up" about the Khazra (Goatmen) etc. This is of course a handy retcon by Blizzard. It makes the text still being canon, they just write about untrue facts.
worth noting that it's a Horadrim mage who writes about it in the D1 manual, and we should really assume they would be more interested in the truth than the Vizjerei. Either Vischar Orous was still loyal to his old brotherhood and knowingly lied, or he was just going through old (incorrect) tomes from the Mage Clan Wars.