Does anybody know if Portal of Praevus addon CD is compatible with GOG Hexen II version?

The answer could be the crucial point in my purchase-or-not decision.

Oh, of course I didn't mean I would install it like that :D

Your post is a thorough explanation, thank you very much.

The upper guide does not work for me, the game will simply boot into the base game. Copying the contents of Install/Data from the original CD will let you start the expansion, but a message pops up you need the original CD (if it is not in the drive).
To my knowledge there are 2 options:

A) use Hammer of Thyrion

B) download PoP rip version from myababdonware.com (I assume this is legal) and copy the contents into the Hexen 2 installation folder. Running GLH2MP.EXE worked fine for me. CD Music does not work with the mission pack.

Hint of the day: Start the game with the demoness, open the console and type 'map demo1'. This will let you play the main campaign with the demoness.

The original installation of Portals of Praevus came with two proper exes called glh2mp.exe and h2mp.exe to run the expansion. In this case the classic quake method of the command line is only functional with source ports. Probably the developers managed in that way to make some kind of drm or copy protection.

So does Hammer of Thyrion contain PoP content? AFAIK HoT is just a source port with graphical enhancements, isn't it?

You can run Portals of Praevus with GOG version of Hexen 2 just fine. And music to play correctly.

1 First, install the GOG version of Hexen 2.
2 Download the Hammer of Thyrion, the main package.
3 Copy Paste its contents in your GOG Hexen 2 directory.
4 Fetch an ISO image of Praevus Portals from an Abandonware site.
5 Mount image and copy only the 3pak file; then paste it inside game directory -> portals subfolder (thyrion creates one, but its own lacks the pak file).
6 Then rename the "Launch" from game directory, the "exe" from desktop shortcut and the executables from start menu, to the aforementioned path, adding -portals as instructed.

After all this is said and done, inside the game, pressing Single Player will lead you to New Mission and Old Mission sub-menus.

7 Finally, go to options, to music and choose FROM ALL CODECS! Ignore midi and cd options. A.L.L. C.O.D.E.C.S.

Enjoy a great experience. Hammer of Thyrion adds a number of nice extra options. I particularly enjoyed the crosshair.

Could you get the CD music to work?

See here:

http://uhexen2.sourceforge.net/music.html

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Hammer of Thyrion does not have original PoP content itself. It Adds two new packs into the data1 and portals folders for its correct compatibility with the games.