OK, I gave up on MageSlayer ever coming to GOG or Steam and made my first ever eBay purchase to grab the CD and manual for it.

Here's what I've done to make the game play on Windows 7.
I have not gotten the game to work with any of the D3D video modes just the default video mode.

Grab the game data from the CD:
1) Place the CD in the drive, cancel autoplay.

2) Copy everything from D:\MAGESLAY into your game's "Install" folder.
ie: C:\Games\MageSlayer

3) Using EAC, rip the CD tracks to MP3 and place in a subfolder called "CDAudio". Name the files TrackXX.mp3 where XX starts at 02.
ie: C:\Games\MageSlayer\CDAudio
NOTE: The MP3s can be placed anywhere, but placing them somewhere else will require you to make changes in step 17.
NOTE 2: I think VBR MP3s have a longer pause between loops. Using CBR MP3s seems to loop better for me. To my ears, the songs aren't designed to loop seamlessly, but have a brief pause between plays.

Patch to 1.1
4) Grab the 1.1 patch from http://www.patches-scrolls.com/dl.php?file=mage11.zip

5) Extract MAGE11.EXE into your game folder and run it.

6) After the successful patch, MAGE11.EXE can be deleted from the game folder

Remove CD check:
7) Open MAGESLAY.EXE in a hex editor (like HxD)

8) Find offset 352F8 and change from 74 to EB.

9) Do the same thing at the same offset in MAGEDX3.EXE

Fix the DirectDraw color bug and enable D3D modes
10) Download ddrawdll.jpg attached to this post into your game folder and rename it to ddraw.dll

11) Download aqritcfg.jpg attached to this post into your game folder and rename it to aqrit.cfg

NOTE: The color bug will still happen in non-D3D modes with this ddraw.dll. If you don't want to play with a D3D mode, see the old steps listed below.

Add CD audio to the game:
12) Download the _inmm ZIP from http://www.geocities.co.jp/Playtown-Domino/8282/
Direct link: http://www.geocities.co.jp/Playtown-Domino/8282/_inmm/_inmm238.zip

13) Extract _inmm.dll, _inmmcnf.exe, _inmmcnf.ini, and _inmmserv.exe into your game folder.

14) Double click _inmmcnf.exe and hit OK on the window that pops up. This loads the default settings into the registry. NOTE: If you have other games that use _inmmcnf.exe, you can keep it in a common location, not necessarily in this game's folder.

15) Drag MAGESLAY.EXE onto _inmmcnf.exe. Say OK to patch the file.

16) Do the same with MAGEDX3.EXE.

17) Download _inmmini.jpg attached to this post into your game folder and rename it to _inmm.ini
This file contains paths to the files extracted in step 3. If you used a different path name, you'll need to open the file in a test editor and change each line.
NOTE: Using relative paths worked for me. Try absolute paths if they don't for you.

Fix 256 Color Mode message:
18) To get rid of the "For best performance, set your desktop to 256 colors. Other settings may cause unpredictable results on some computers." message, right click MAGESLAY.EXE and go to properties. Go to the "Compatibility" tab, then place a check next to "Run in 256 colors"

19) Do the same for MAGEDX3.EXE.

NOTE: Changing this setting made the load time increase for me. I prefer not to set it.

Run MAGESLAY.EXE to play the game.

OLD STEPS
Fix the DirectDraw color bug
10) Grab ddwrapper from http://www.bitpatch.com/ddwrapper.html
Direct link: http://www.bitpatch.com/downloads/ddwrapper.zip
Extract aqrit.cfg and ddraw.dll from ZIP and place into the game folder.

11) Open aqrit.cfg in a text editor. Change the line for ColorFix so it reads
ColorFix = 1

I still get the color bug sometimes. Sometimes it corrects itself when going to the menu or starting a game. Also try Alt+Tab and if all else fails: save, exit, restart, and reload.

Doesn't Alt+Tab fix that?

It might. In the times when this happens, the colors sometimes fix themselves when I get to the menu. I'll try Alt+Tab when it happens again to see.

EDIT: Alt+Tabbing back into a game caused the color bug to start happening, so I tried Alt+Tab and that didn't fix it. I started a game and the colors fixed themselves.

I see. When I used ddwrapper with the first Diablo, it would ocassionally get the colour glitch as well, but when I alt+tabbed and returned, the glitch was gone. I guess it might differ depending on the game in question.

It might work sometimes. It can be finicky.

I think I have the D3D modes working! (I also had to add a step for _inmmcnf.exe)

Using aqrit's ddwrapper source code found at https://github.com/aqrit/ddwrapper , I made the change that aqrit gave in a discussion on
DxWnd at SourceForge (modified slightly to fit the github code). Once I found the DirectX 7 SDK header files and added a couple of missing lines, I was then able to compile the DLL and use it.

To ddraw.h:
#define DDSD_DEPTH 0x00800000l
To dvp.h:
DEFINE_GUID( IID_IDirectDrawVideoPortNotify, 0xA655FB94,0x0589,0x4E57,0xB3,0x33,0x56,0x7A,0x89,0x46,0x8C,0x88);

Note that with this DLL the color fix doesn't seem to work with the non-D3D modes, but I haven't experienced the color bug in the D3D modes (and the CFG files doesn't seem to be needed or even used with it...).

I played through to the sewer boss level and there did not seem to be any problems.

Still verifying things, but they are looking positive.

I don't have Take No Prisoners, which uses the same engine, but this file might also work for it.

Tried DXWnd? That might help.

Tried it and didn't get it to work for me despite it having a MageSlayer export set up.

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Comparison screenshots.

One in 640x480x8 and the other D3D 800x600x16.

Just bumping this thread to say that if nothing else works, use DGVoodoo 2 to run this game, because that worked perfectly for me.

Fun game, still have my old physical copy but haven't tried it on Win 7. Thanks for your efforts and for sharing these tips. +1

I can't seem to find the hex offset 352F8 that you're talking about...

I'd love to get this game running with CD audio :D

EDIT: Turns out with mine, the offset was 352F0 in case anyone else reads this and has an issue there.

Great guide! Thanks for taking time to type it up!!

I'm off to slay some mages! :)

Was it worth the trouble?
It's not excactly remembered as a great game...

Wow that was complicated... I got Mage's Slayer's cousin : Take no Prisoners working that way - Use virtual machine for win98 to install it properly, then simply loaded the game using win7 and it worked like a charm.

Did you patch the game first?