Excerpt from Denizens and Dangers of the Outer Planes. Published 2368, Banned 2370,
There is a danger to be found in all kinds of magic.
It is common knowledge that not everyone is capable of challenging the divine magic of the gods. If one lacks the proper biology and attempts to manipulate divine power, usually through an item infused with such, it will instead burn out their body and soul, leaving them an empty and broken husk.
Less severe are the consequences of attempting to cast arcane spells above your skill or energy level. The most likely outcome of an apprentice, or even journeyman, Mage attempting to cast a spell they are not ready for, is that the spell will fizzle. Almost as likely, however, is the possibility that the spell will ignite and quite literally explode in the young Mage's face. Even should the spell succeed, no matter how slim the possibility, there will still be consequences to meddling in such advanced magic. To power itself sufficiently, the spell will pull its energy from the Mage, burning out the channels in their bodies and brains required for spell casting, possibly destroying their ability to cast any magic at all in future.
All of these pale in comparison to the fate of anyone who attempts to cast magic originating from the Outer Planes. In other words, Warlocks. I have spent the last three decades studying the history of individual Warlocks, and my findings have served as proof of the danger to both themselves and anyone around them.
It starts with their bond. Much like Divine spell casters, Warlocks are given their magic by a Patron, a denizen of the Outer Planes, the nature of which I will discuss in detail starting in Chapter 2. Unlike divine spell casters, it seems that NO human is capable of channeling this type of magic without feeling the ramifications in their bodies and minds. Dispite this, and unlike the gods, these Patrons do not seem to care and will grant their power to anyone foolish enough to ask.
The magic itself is a perversion of both divine and arcane spells, able to manipulate life and the elements in a way that twists and mutilates. What should create healing instead creates rot, and what should manifest in typical damage instead poisons the very land and body the spell is cast on, tainting the natural cycle of death breaking down to feed new life.
However, all of this is secondary to the effect the power has on the Warlock's mind. There are countless cases of good, though desperate, people forming a Warlock bond intending to use the power for good. In every case, the practitioner may spend a few years doing just that, but inevitably they all fall to madness.
Some begin to hear voices; others lose all connection to the real world; some become clinically paranoid; most lose the ability the empathize; and all of them eventually turn on even their original noble goals. Often they go so far as to kill their families and anyone else unfortunate enough to be around them. Either in their paranoia, an effort to gain more power, or simply because they got in the way and were no longer recognized as sentient beings...