How to run Rogue Legacy for Linux on a specific monitor?

Here are some things to try:

If the game uses SDL, it is possible to set which monitor fullscreen games will run in by setting an environment variable like so:

export SDL_VIDEO_FULLSCREEN_HEAD=0

Which you could stick into your preferred userland environment file, depending on your flavor of linux and shell.

or maybe

Adding the following line to /etc/environment.

SDL_VIDEO_FULLSCREEN_DISPLAY=0