Novel involving a powerful spellcaster trapped at the bottom of a well or pit. The magic of the pit kept them from casting spells

Question

I'm trying to find a novel I read as a kid.

Key things I remember about the book:

• I found it at a library ~15 years ago.
• I believe it was a dark/horror fantasy.
• It may have been set in a tabletop role-playing game setting (like Forgotten Realms, or one of those), as I read a lot of those.

The main thing I remember was a particular enemy:

• They were some kind of spellcaster trapped at the bottom of a well or pit.
• They were incredibly powerful, and capable of immense destruction with only a word.
• The magic of the pit kept them from casting spells.
• Their attempts to escape were a key part of the plot.
• At one point, the magic of the pit began to weaken, and they began sending up evolving constructs that, if left unchecked, would gain the ability to cast spells and free the enemy.

Thanks for the help.

Answer 1

Possibly Peril's Gate by Janny Wurts.

This is part of a rather sprawling series that I only vaguely remember. The series starts with the defeat of an entity called the Mistwraith or Desh-thiere. The Mistwraith is defeated and imprisoned in Rockfell Pit by the two brothers Arithon and Lysaer. Though defeated, the Mistwraith curses the brothers so they are doomed to eternally fight each other and the following countless books detail the conflict between the brothers.

Anyhow, in Peril's Gate the wards at Rockfell are starting to break down and the Mistwraith is on the point of escaping.