Lament of the Void

Lament of the Void is probably the most mysterious book in the world, believed to exist merely in two copies (one of which was destroyed, and the second stored safely in the Vault beneath the Imperial Library).
There is not a single thing known for sure regarding this book beside the fact that it exists and is under Damnatio Memoriae treatment of Imperium. Meaning that its contents are considered damnable, knowledge about it is actively suppressed, and all additional copies are supposed to be incinerated immediately after discovering, with their ashes scattered.
It is believed to be the most eldritch and terrible book connected to Pentagram, but exact details of this are unknown. Mostly due to how succesful is Imperium in purging knowledge about it.

Purpose

Unknown. Noone knows who wrote it, why wrote it, or what exactly wrote in it. Some myths suggest that it might be somehow connected to the mysterious fifth Imperial God of the Pentagram.
In fact it is so mysterious, and Imperium is so succesful in purging it, that there is a theory that there are in fact no such book, but some sort of an on-going magical effect that persuades people that such book exists. Since pretty much every magician knows about it, while not knowing what it is.

Historical Details

Background

Exact background for this book is actually unknown as well. Imperium thoroughly purged everything even remotely connected to the Lament, including (according to some myths) literally wiping out entire region, not only in a literal sense but also by editing all sorts of documents to completely purge it from existence).

Legacy

It is well known as a sort of archetype of a tome filled with terrible secrets and knowledge, even if noone really knows anything about it.

Type

Manuscript, Magical (Tome/Scroll)

Medium

Paper