I know there's already a thread bout this, but most of those links are dead, so I'm making a new one (minus the DXX-Rebirth stuff because I'm not as familiar with the source port).

Linked below are some ZIP files for your GOG install that will re-implement some of the missing content for the first two Descent games that were included in the Definitive Collection release of the game. Just extract their contents to their respective directories and it should work fine.

Descent 1 Bonus Levels + Levels of the World
Update 10/19/23: Included a copy of DOSBox Staging in order for the folder overlaying to work properly.
This patch includes:
- 20 Extra Missions by Parallax Software
- Descent: Levels of the World - This add-on has more than 100 Descent levels created by fans and judged as the best by Computer Gaming World Magazine in a contest plus an all-new level from the creators of Descent. (Description from Moby Games)

DOWNLOAD

Descent II Vertigo Series + Alternate Soundtracks
This patch includes the Vertigo Series Expansion (with working music), as well as the ability to choose between the original, Infinite Abyss versions of the base game soundtracks. For more info on the alternate soundtrack versions, read this (copy-paste it to your address bar directly otherwise it won't load):
https://web.archive.org/web/20191012100242/http://www.descent2.com/goodies/music/d2_tracklist.shtml

DOWNLOAD

Descent Mission Builder II
The original Descent level editor, as bundled with the Descent II Vertigo Expansion. I believe it was designed to work with both Descent 1 and 2. This is patched to the latest 2.6 version.

DOWNLOAD

NOTE: Unfortunately, Descent Mission Builder II doesn't work on modern windows in my testing, so this is just here for archival purposes. If you find a way to get it working, let me know.

If there's anything other missing official content I should include, comment below. :)

Would D<MB2 not work in XP mode on your computer?

Tried pretty much all the compatibilities from XP SP3 all the way back to 95, none of them seem to work.

Can I not just throw this content at DDX Rebirth?

Updated Descent 1 patch, see main post for info!

Maybe? I'm not super familiar with DXX Rebirth so I can't say for sure.

Yes, you should be able to use this with DXX-Rebirth. DXX-Rebirth is a source port intended to make the original games work on modern computers. Rebirth should be able to read and play original retail levels (as sold by GOG), contemporary expansion levels (Levels of the World, Vertigo), and any more recent levels made by the community (except perhaps those that specifically target DXX-XL, which is much freer about adding incompatible features, and so may accept levels that neither GOG's DOSBox Descent nor DXX-Rebirth will accept).

Descent Mission Builder II WORKS without problems if you run it on Linux via Wine. I'm using Fedora 39 and Wine version 8.19.

At least it opens up, you can use the controls and can load the tutorial save file. I dont quite understand how it works, though. It displays some kind of wireframe graphics on screen, but I don't know how to go on from there.