You need a copy of Chaos Legion for PC. Since it is abandonware (no longer sold on Steam or GOG), you will likely need to source it from an archive. Ensure you are using the "English/European" release, as the Japanese release has different executable offsets.
This is the most important part. Sometimes the game launcher or the game itself will try to overwrite your changes upon exit, resetting you to a lower resolution. chaos legion pc widescreen fix
Launch the game via the new launcher, not the original .exe . Once in the game’s main menu: You need a copy of Chaos Legion for PC