There's a good site about Darkstone.
I found questpack and HD Textures in this site.

Here's the Pack:

Download 1 (Mediafire)

Download 2 (Pixeldrain)

It contains All custom Quests and HD Texture Pack and Widescreen fix.

For Quests just copy to "Quest" folder directly (without folders). But for HD Texture Pack You must use dgVoodoo2. And unpack texture pack to game directory. Here's the tutorial https://www.youtube.com/watch?v=9IfXSpdKgC8 - how to setup dgVoodoo2.

Edit:
Links update.
23.09.2025.