Calling the Patronus

Although some Fallen Ones are intuitive enough to learn the Arts on their own, most require an instructor of some kind. If our Infernalist has joined a sect, he’ll receive a mentor (often in exchange for some hefty favors and/or servitude); even if he studies alone, chances are he’ll attract the attention of a mentor before long. Aside from the details — which vary from teacher to teacher, but tend to be fairly grotesque — an Infernal apprenticeship is much like any other. An older Infernalist takes a less-experienced one as a student, the student performs some degree of service, and the elder passes on her secrets. Or most of them, at any rate.... Sooner or later, the Infernalist wants more. Perhaps he’s too proud or impatient to withstand an apprenticeship; or maybe his mentor is too perverse even for a young warlock’s jaded tastes; worst of all, they may have had a falling-out.., usually the prelude to an all-out witch-war. When mortal teachers fail, or when there are none to be found, the Infernalist calls upon a demonic tutor and offers him some favors in exchange for knowledge or power. As risky as it is, this step usually comes early in an Infernalist’s journey; the Fallen One meets his Patronus (patron) during his renunciation ceremony. (The demon has, more often than not, been watching since the beginning, but only reveals itself at the initiation.)
At some point, the aspiring demon-servant gains a ritual that allows him to call upon an agent of the Pit; if he’s wise, he’ll use it when he has the strength and experience to handle a demon face-to-face. Most warlocks are too proud, desperate or impatient to wait until then, however; some die or go insane the first time they summon a demon to their aid. Admittedly, the raw fury of a demonic manifestation is enough to send even an experienced wizard sprawling. To invoke an evil spirit, the Infernalist must perform some appalling ceremony and make the appropriate sacrifices (usually live, almost always messy). The demon makes an impressive entrance, throwing minor tempests in all directions and often indulging in some hideous display for sheer shock value. (Considering how jaded most Infernalists are by this point, that kind of display is best left to one’s imagination.) Howling threats or offering sweet seductions, the evil spirit assesses the Infernalist’s reaction; if he seems to have potential, the bargaining begins....
MostInfernal pacts are sealed this way: highly symbolic offers are exchanged until both parties get what they want. The Infernalist inevitably has to commit some sinful, often hazardous act, and the demon offers some minor but enticing trinket. Most demons snare their quarries here; many mortals, unsure of what they want or how to ask for it, give away their souls for very little profit. Some Infernalists sprout hideous Investments and become virtual devils themselves, but rarely realize how much they could have had. A warlock who masters the steps before this dance begins often gains an impressive Patronus... even as he becomes its slave.

Many others, confronted with a soul-seller, wonder how on earth someone could barter away both soul and freedom. But the Infernalist sees things differently. To him, the demon is not his master, but his advocate; the Patronus has mastery over certain things, and the warlock helps him keep it, in exchange for a few choice favors. This illusion — and it is an illusion! — of partnership keeps the Infernalist from seeing the abyss at his feet. He may feel its heat, but he’s convinced that he can either fly over it or escape it at the last moment.
(Infernalism:PoS 42)