At the end of the game, upon defeating Hessonite, you are given control of the Prism and can make choices. The choices you need to make for this achievement are "No!" and "I want to go with Hessonite" when you choose those options, let the cutscene play, and your achievement will unlock. (If you have already beat Hessonite, don't worry! You can go back and fight her as many times as you want)