In his Going Through Forbidden Worlds, Zzarchov Kowolski leaves it to the GM to create several demonic encounters for the adventure itself. For doing so he points to the Summon spell (LotFP Player Core Rules p. 134) and/or the Random Esoteric Creature Generator while also giving the advice that many of the creatures encountered may be influenced by a catholic´s vision of hell and the demonic.
What follows is an independent production by Gregorius21778 and is not affiliated with Lamentations of the Flame Princess. Lamentations of the Flame Princess is a registered trademark owned by James Edward Raggi IV.
I am not a catholic and I don´t own a copy of the Random Esoteric Creature Generator, but I am gifted with a queer creativity, a knack for handling an RPG´s rules and tables and (currently) a LOT of time at hand. So I did some brain storming and crossed-bred the results with the tables of LotFP Summon spell for reference, further inspiration and right-sizing. Out of it came 10 demonic beings. Eight of them are HD:3, the last two are HD:6. You do not need Going Through Forbidden Worlds to make use of them, they will work in any other LotFP game as well. Here they are, as a PDF straight from my dropbox.
Have fun with them, and feel free to leave a comment to tell me how my creations fared in your game. Its is always nice to hear from “the children”.
EDIT: Version 1.1 is now available under the link above. The demons are still the same, but I had to correct some errors in the text and “missing parts of the sentence”.