Avernic book

The Avernic book is an item reward from the Demon Slayer quest. Players who completed the quest before the item was added can speak to Gideon Bede to obtain it.

It requires 1 Magic like the wizard's book, but has noticeably higher accuracy. Unlike the avernic wand, it does not provide unlimited air runes.

Transcript
Preface

I record what I have learned of demons, in hopes that their forbidden secrets be used against them.

Be warned - I cannot assure the authenticity of this document, for much of my information was gained from demonic tongues.

Do not be tempted to abuse this knowledge for power or glory.

I pray that your faith is strong, and your mind protected by the holy might of Saradomin,

Relenor of Varrock

The Many Faces of Evil

The forces of darkness are legion, and myriad of form, for they are creatures of chaos and madness.

I was able to secure one of the lesser demons in a binding circle and, though it took much encouragement, extracted some critical information before my ministrations reduced the creature to ash.

I forced from its scaled lips the name of its kind - Avernic. It is my belief that this is the true name of demonkind.

Cherish this knowledge, for as it is written, the true name of demons gives us power over them.

Pandemonium

I was able to secure other knowledge from the demon, before its wounds proved too grievous

The Avernic dwell in the Infernal Dimensions, known also by its secret name of Pandemonium.

The largest city on Pandemonium is called Dis, and the city is ruled by a massive Elder Demon know in their infernal tongue as Gree'gar Tsutsaroth.

This Elder Demon is Regent to the dark lord Zamorak, the arbiter of destruction and our greatest adversary.

The Heart of Darkness

Emboldened - some would say foolhardily - by my success, I sought out other sources of information on demons - or the Avernic, as I now knew them.

Though it took no small persuasion to secure permission from the Tower Wizards, I interrogated Azacorax, an imprisoned lesser demon, and learned more of Pandemonium, the 'place of all demons'.

The heart of Pandemonium is an eternal, burning fire.

From fire all demons are born, and to the fire they shall return.

The flame of Pandemonium burns within each demon, sustaining their vile thirst for destruction.

Demons, you see, are unnatural creatures, and the wheat of the field and the flesh of cattle smoulder within their foul mouths, providing no sustenance.

Manifestations

On Pandemonium, the Avernic have a physical form that encases their heart of fire. A demon in its true form is nigh unstoppable.

It is possible for a demon to travel the planes and carry their full might to another world, with great difficulty and risk of permanent destruction.

The Avernic, cowards and heretics as they are, prefer to project a manifestation into our world from the Infernal Dimensions.

The demon constructs a temporary form or possesses a living host, in order to claim a foothold in Gielinor.

Should the manifestation be killed, normally the body shall crumble to ash, and the demons consciousness returns safely to Pandemonium.

Temptation

The wizards would allow me no further contact with Azacorax. They warned me that such knowledge is dangerous, but I am compelled to delve deeper.

I have booked passage to Karamja, where it sis rumoured one of the greater demons dwells in the deep jungle.

I intend to confront the demon, discover its secrets, and write another account upon my safe return.

With the power of Saradomin on my side, how could I fail?

Trivia

 * The inventory icon is the same as the Malleus daemoniorum.
 * When in a player's bookshelf, its named "Trivium Daemonum".
 * It would appear that Relenor either died or disappeared after encountering Nezikchened. He, or his remains, have not yet appeared in-game though.