https://bugs.winehq.org/show_bug.cgi?id=29521
--- Comment #34 from padraig padraig_mac@hotmail.com --- Just a further update for how the texture mirroring can be fixed. Download the ddraw.dll and config file from here: http://www.bitpatch.com/ddwrapper.html
Put them in the same directory as EngRel.exe and in the config file set BltMirror=1
Then use winecfg to add ddraw to the overrides library as native first and then the textures will render correctly.
I should also point out that everything I've said doesn't work with the original executable, you need to use the hexedited one which is one of the executables in here: http://www.mediafire.com/file/fnnkq89xdjy75m0/DarkOmenMods.zip/file
(There are other ones around dark-omen.org too but I haven't tested them all).
The game will run perfectly then in Xephyr with 16 bit colour.