From publisher blurb:
Fascinating and Terrifying Elements for Any Fantasy Game Where the Heroes Encounter the Fiendish
Designed specifically for people playing 5e campaigns.
Continuing the combination of utility, imaginative content, and deep research found in such books as Treasure Discoveries, Feywild Discoveries, City Discoveries, and Draconic Discoveries, Evil Beagle's resident scholar and master-of-cleverness, Michael Surbrook (AKA "The Professor"), brings another useful toolkit to bear for Game Masters who want to elevate their games. In this case, Fiendish Discoveries is designed to present game masters with a full understand of and extensive resources for the use of fiends in a campaign. This includes:
The Nature of Fiends: Discussing the very nature of demons, devils, yugoloths, and related fiendish spirits. This includes understanding their psychology, how to roleplay them, what they want, and other ways to believably incorporate them in your campaign.
Fiendish Patrons: A keystone part of playing a warlock, a patron is an intricate influence on a warlock PC's experiences. The nature of the fiendish contract for patronage is explored in great detail.
Fiendish Bloodline: Did your sorcerer enter in a fiendish contract for power, or did an ancestor have a... troubling relationship? The benefits of such a background for a sorcerer are detailed in this new PC option.
Fiendish Portents: Sights, sounds, smells, and other indications a GM can use to enhance the flavor and experience of a fiendishly-entangled storyline.
Fiendish Encounters: Numerous set pieces - Gates, Lairs, Scenarios, and more - designed to drag-and-drop right into your fantasy campaign to add the terror and mystery of the fiendish to the story.
There's also a selection of Asian/Pacific Rim fiends to compliment your collection of monsters found in standard D&D sources. As well, the Professor's compiled a very thorough Fiendish Reference index, indicating what fiends can be found where among those same sources. Finally, a complete (and completely horrifying-yet-fascinating) set of randomizing tables enabling you to generate a nigh-infinite variety of unknowable fiends for your campaign.
Fiendish Discoveries is an invaluable tool for any GM hoping to dramatically enhance the presence and quality of fiends within their campaigns.