I was reading the essay on extraplanar life from Fredrick Modah/Armifer and noticed that no one had brought up here the discussion of demons and necromancy in the work.

"Arguably the most important kingdom to understand in the past few years is, unfortunately, also the least understood. It is often theorized that all demonic life comes from a shared home on a unknown plane, but it is more accurate to simply say we have no clear idea where these creatures come from."

"All creatures we classify as demons share a common feature: they are inevitably malevolent and destructive. From our limited understanding of demonic life, the Plane of Abiding appears to be a source of nourishment to them, and they regard our very existence in the same way we would look upon a fruit hanging just out of reach. Visually they share qualities which are often described as "nightmarish," either bizarrely organic or darkness shrouded in a way that can be visually confusing with lesser conceptual life."

"Demonic life has a connection with necromancy and the undead which is poorly understood. While not all undead are of demonic origin, it is commonly thought that demonic patronage is required to perform necromancy, as the source of reality-defying "anti-life" which powers the mad art. However, we will be forgiven (and grateful) that we are unable to cite any "experts" in this field."

Thanatology clearly plays a role in the confounds of necromancy, but I think that the background has been purposefully vague on what's at work behind the scenes. There's no clear statement on what makes Thantology rituals work. A Necromancer could just as well believe that they hold the secrets to a "purer" or "stronger" form of magic because they don't have to rely on the aid of denizens from another plane. A number of Philosophers seem more concerned about spiritual possession and demonic influences with incorporeal necromancy than the potentially unseen forces aiding them in their Great Work.

The possibility that demonic denizens are aiding Necromancers in their ceremonies would be unsettling to most Philosophers. A Necromancer who believed that demons were an aid would have to address some uncomfortable questions about why they are so helpful. I could see a more Perverse Necromancer being fine with it because both the demon and the Necromancer are out for the same thing, assorted malevolence and destruction. But has anyone thought of other ways to roleplay the relationship between their Necromancer and potential sources of power?

just like books and pretty much everything thats been written IG are subject to perspective. it was admitted that the knowledge of the demonic plane was sketchy by the thone folks.
top level philosphers might have demonic interplaner travel or communication down to a science. And if a demon can control one of the thirteen, whose to say it can't go in the reverse?