Translates D3D9 games to D3D12 using Microsoft's own D3D9On12 wrapper library.
Windows 10 Insider Preview 18956 or later is required for this to work properly. (and an SDK >= 18956 to build)
Just download the latest release,
copy the d3d9.dll next to the executable of the game you want to run, and start the game.
Make sure to use the x64 one for 64-bit games, and the x86 one for 32-bit games (you probably want the latter).
If everything works correctly, you will most likely experience reduced performance compared to native D3D9, and in some cases rendering issues and/or driver crashes. Some games may refuse to run entirely.
Note: Tools like MSI Afterburner will still show D3D9 in their overlay, this is because D3D9On12 is implemented using DDI.
Why not? Just don't expect any sort of support when running into issues.