You can gather from their unspeakable arts what the malice of these hellish spirits is like. There are so many of these arts that listing them would fill a book, and describing them would take volumes. Most of these arts are unknown in the world. One kind has to do with the misuse of correspondences; a second kind has to do with the misuse of the most superficial features of the divine design; a third kind with the sharing and instilling of thoughts and affections by distractions, focusing inward, and the use of decoy spirits as well as by emissaries; a fourth kind with manipulation by hallucinations; a fifth kind with external projections that enable the spirits to be present outside their bodies; a sixth kind with various types of imitation and persuasion and pretense.
The spirits of evil people come into these arts spontaneously when they are freed from their bodies. These arts are inherent in the evil they are then absorbed in.
They use these arts to torture each other in the hells. However, since with the exception of the different varieties of imitation and persuasion and pretense these arts are unknown in the world, I do not want to describe them in detail, both because they would not be understood and because they are unspeakable.
The reason tortures are permitted by the Lord in the hells is that there is no other way evils can be restrained and tamed. Fear of punishment is the only means of controlling and taming evils and keeping the hellish mob in restraints. There is no other way, for if it were not for the fear of punishment and torture, evil would plunge into rage and destroy everything, as would happen in a kingdom on earth where there was neither law nor punishment.
from Heaven and Hell, Sections 580-581