Alright so I can definitely confirm that this achievement is MISSABLE!
It is directly tied to the quest "Free Their Minds, the Rest Will Follow". You can complete the final act without finishing this and then the game automatically removes it from your quest log.
Thankfully I had an autosave that allowed me to go back and replay a little bit. Once you complete the above side mission the achievement will pop at the end of the conquest.
Good luck! Happy Hunting! Any questions let me know; hopefully this saves your play-through and your time.
