"Still one of the best melee 1st person combat system ever..."
(THIS GUIDE IS FOR THE SINGLE PLAYER GAME)

NOTE: this guide doesn't cover crash on startup or other launch issues a very few users are reporting, but is to fix silent crash on quit or in-game instability. See the Settings section in case... or write in the generic community thread. THIS IS NOT THE RIGHT PLACE TO ASK FOR HELP IN THAT CASE.

The game always crashes when it has to quit back to desktop (either a silent crash visible only from Reliability Monitor and Event Viewer or a requester with the classic "mm.exe has stopped working" error).
Steam version can also randomly crash while playing or when loading new levels...

July 2019:
Added a few benign patches/tweaks (see at the bottom) solving game stability issues.

N.B.: the Steam version uses a buggy build 168.62579 (master 1.02). The retail DVD version + 1.02 official patch 175.52126 (master 1.02) is newer, more stable and recommended. But you can't apply the retail patch to the Steam version. Thanks to this guide the Steam version will became just as stable as the retail version anyway...

(Small donations 💰 to buy and fix some more games are welcome from my profile page)

📢 Before you apply the steps below, be sure you removed any exe compatibility setting you might have changed (run as Admin, XP mode etc.), and remove any Steam launch option if present. Be also sure the game is forced to run on dGPU and not iGPU.

(Hexaae, July 2019)
These Compatibility Fixes* seem more reliable under Windows 10 1903 with improved performance:

• HeapValidateFrees
  
• IgnoreCRTExit

* See for example my other guide with instructions on how to install and use Microsoft Application Compatibility Toolkit to manually apply the fixes.

Simpler solution: here is the DMfixes.sdb file for those lazy or newbie users scared by MS-ACT:
https://1drv.ms/u/s!ApMUGr0cuN39hIlt25hat56_58w57Q?e=I5AycO
Download (it's just a OS-wide compatibility patch data file specific for this game) and apply it from an Admin CMD prompt, like this:
You can then uninstall this fix at any time from the list of Windows installed programs.

It should reduce loading crashes.
With an hex-editor you can patch the game (mm.exe) to be able to use >2GB:

Go to byte position $DD6 (=3542 in decimal numbers) and change the byte $0F to $2F

If you're too noob to manually modify the exe, you can try with tool 4GB Patch: https://ntcore.com/?page_id=371

This seems to be a benign setting improving stability against unexpected crashes while playing.
On modern systems with 4GB+ RAM and 2GB+ VRAM, set as Steam game launch option:

+datacachesize "128"

SKIP INTRO VIDEOS
Add this game launch option:
-novid

🚨NOTE: unfortunately this old game seems to have some issues with modern multicore cpus, resulting in random minor stuttering and non-perfectly smooth experience because it won't distribute cpu work over all free threads. There is no known fix ATM, but as a partial workaround you can try hitting ESC (pause menu) multiple times and see if this improves smoothness...

PARALLAX MAPPING
Not sure if it this actually works (should enable parallax mapping for an improved real 3D bump effect for some texture?), but you can try opening registry at HKEY_CURRENT_USER\SOFTWARE\Valve\Source\mm\Settings
and modify:
mat_parallaxmap = 1

Warning: this can cause the game to crash according to https://developer.valvesoftware.com/wiki/$parallaxmap

LAST RESORT
If the game still crashes unexpectedly while playing, the last thing you can try is to set through NVidia Profile Inspector or NVidia driver settings: Threaded Optimization: OFF.

If you have problems with settings that won't be saved on exit, be sure your Windows registry keys for this game are ok. In particular check under:

If you have multiple entries like these:
make a backup and DELETE them as they can interfere with Dark Messiah which can be confused mixing all these different settings together (!).

In case, to reset everything 'till next game launch, you can delete the whole "HKEY_CURRENT_USER\Software\Valve\Source" registry key, which will be re-created on first game launch.

Original (lost?) post: http://forums.steampowered.com/forums/showpost.php?p=28043316&postcount=1

For further issues with old and unsupported Windows 7 (e.g. the game is not starting/black screen), please read this: https://psteamcommunity.yuanyoumao.com/app/2100/discussions/0/558747922227774495/?ctp=5#c1747897731973522665 (in brief you could try adding the Compatibility Fix "IgnoreLoadLibrary" (-> gameux.dll as Parameters) to avoid mm.exe hanging at launch in Win7)

引用自 thepassengersep：

Hello all.
Thanks to the efforts of a GOG community member I was able to obtain a fix for the crash on exit that occurs on Windows 7 64bit.

I have copied the instructions over from the page below; but as the fix was originally meant for PlaneScape: Torment, I have made the proper adjustments:

http://www.gog.com/en/forum/planescape_torment/windows_7_64bit_fix_for_crash_on_exit_screen_shaking_flickering

1.) Download & install the Application Compatibility Toolkit Setup from here
https://docs.microsoft.com/en-us/windows-hardware/get-started/adk-install

NOTICE: You don't have to download 4GB of full ADK package: it will run a small Microsoft downloader from which you can select only the required tool (less than 15MB)

2.) Run Compatibility Administrator, the 32bit version (since mm.exe is 32bit).
It will start with New database(1) [Untitled_1] highlighted.
At the top, click on Fix.

3.) Name the file to be fixed ( I put Dark Messiah ). Then browse to your mm.exe file.

4.) Click next, to the compatibility mode screen. Ignore that & next again.
At Compatability Fixes, tick the ones you want.

EmulateHeap
IgnoreCRTExit
IgnoreFreeLibrary

were enough to stop it crashing on exit.
You can click Test Run to make sure.
Then next, then Finish.

5.) Highlight New database(1) [Untitled_1]. Above click on save.
I saved my file in the same directory as mm.exe and named it Dark Messiah Fix.

6.) Then right click on New Database(1) & install.


Here you can find the descriptions for each flag I used:
https://technet.microsoft.com/en-us/library/cc765984%28v=ws.10%29.aspx (updated Jul 2017)

I hope this helps those whom are bothered by the crash on exit, and I will post again if I discover any undesirable effects.

But in the meantime it works rather nicely.

Thanks and credit goes to torarngu & olnorton of the GOG community for original post and clarified instructions respectively.