Throne of Night, the failed Kickstarter by con-artist Gary McBride does actually have some amazing low level under dark stuff in (for levels 1-7 which neatly covers the area you would normally spend above ground in Night Below.)
Highlights include a drow outpost, a fungal jungle, a Myconid city, a more interesting Svirfbeblin village than in Night Below with maps etc. A magical manifest zone, an underground forest full of dinosaurs and plenty of other cool stuff. It’s a tragedy that it wasn’t finished.
As much as it pains me to recommend that you send money his way, it really is one of the best underground adventures I’ve seen. And perfect for strip mining for ideas, encounter, art etc.