Focus energy can drop as a completion reward for completing nightmare missions, If you are running normal nightmare missions then you are going to be running missions on Uranus, Pluto, Eris, Sedna, and Neptune as you can get C rotation rewards from running nightmare mission on those planets.
The other mission type you can run is nightmare rescue missions however there are 3 objectives you have to complete during the missing in order to get rotation rewards those being:
- You have to save the hostage undetected
- You have to save the hostage
- You need to kill all the wardens
You can run the nightmare rescue missions on any planet also.
Do know that mod like these do have significantly lower drop rates so you will most likely be farming for the mod for A while. (I wasn’t able to get the mod in my run, unfortunately)