The Tome of Khainan

A text written entirely in infernal, with illustrations of devils, fiends and unholy rituals.
The text makes up a combination of both a grimoire and a journal, speaking of past events and machinations of an infernal cult. Within the opening chapter, the text speaks of an eternal war between demons and devils, but not within Avernus or the Abyss but a demi-plane where those who have fallen and failed have been sent to eternally prove themselves worthy to reside amongst their overlords once more. A prison of violence and eternal suffering where the weak die and are never reborn, where the weak become strong in their victory. The Plane of Zorhenna said to be a place that even devils and demons fear, one created by Asmodeus for the insolence and neglect of his servants and adopted through trickery to be shared by the Lords of the Abyss.
In the following chapters, it speaks of Demons and Devils rising greater in power than dukes and lords of their former homes. Seeking worshippers and followers on the material to raise them in their status and seek power to not regain their former purpose, but revenge instead. Turning their backs on their former masters, Zorhenna would not longer be the playground of fallen fiends, but a training ground of betrayers and entities seeking retribution and power beyond their former comprehension. Fearing their growing power, some were not even allowed their escape, despite the deal that was struck.
Worshippers of these fallens fiends would communicate as best they can with masters, doing everything in their power to find a way to release them. The Abyss and Nine Hells were not longer home, and so these fanatics would seek to bring them to material plane. But it was clear that the laws of Zorhenna would not permit a fiend to leave... but in another form would bend the laws to allow them. Using their followers as vessels, the worshippers would offer themselves in their entirety to see them freed. They would live as mortal which great power until they could see their true capabilities unleashed.
Thought few tried, and many failed. These greater mortals would be known as fragments, for they were merely an essence of their former power. It does not, however, state whether any succeeded in releasing themselves from their mortality and finding their fiendish form on the material planes. And of those that did succeed, they will riddled with eternal madness for they were unstable conduits of the fallen fiends.
Towards the back of the grimoire, spells were listed in detail of their summoning and ritualistic requirements.
Spells include the following;
Hellish Rebuke
Darkness
Enthrall
False Life
Contact Other Plane
Infernal Calling
Summon Fiend
Summon Familiar