Although T. H. Lain's books frequently don't reveal the names of the monsters and spells they feature, they seem to all be sourced directly from rule books, and I've generally been able to use the novel's descriptions to figure out what they are.
However, I had no luck with two of the monsters in the book "Oath of Nerull".
The first was some sort of inky oily tentacle-thing that announced it's presence with a musical flute-like piping. Based on the party's borderline irrational reactions, it might be able to cause some sort of fear effect. It might also be worth noting that the specific one in the book was capable of speech, allowing the group to avoid a fight by returning an object they had accidentally stolen.
The other one was occasionally referred to as an "Abyssal Child", and was some sort of combination slug/infant summoned by cultists. It was also capable of speech, and had skin acidic enough to cause injury on contact.
Nothing relevant or helpful seemed to be in the Monster Manuals I browsed, so either I missed something or they're from a source book that also covered other stuff. Since "Oath of Nerull" came out in September 2002, I'd assume whatever they're from came out around that time or earlier.