Monstrum, or A Portrayal of Witchers: Volume 2

For it is well known that when a witcher inflicts torment, suffering and death, he experiences a semblance of pleasure and delight, the kind a normal and righteous man only feels when performing his marital duties with his spouse, ibidem cum eiaculatio. From this it clearly follows that the witcher is, in the very matter of his being, a defiler of nature, an immoral and loathsome degenerate, born from the darkest and rankest depths of hell, for only one such as the devil himself can derive pleasure from suffering and torment.

[note in margin]

The book is a key. The mines near Kaer Morhen.