Your general needs to eat three skulls. Skulls are left on the battlefield when you kill an opponent, and eating them heals your units and increases their maximum health. Eating three of them will turn any of your units into a demon, which gives them extra actions each turn. This achievement can be unlocked on any level in any game mode. If you do the first level which serves as a tutorial, as soon as you are told about eating skulls you can unlock this achievement. Simply select your general and have him eat three skulls over three turns, retreating behind your line of infantry after doing so to prevent the AI from attacking him. The achievement will unlock after eating your third skull. If synching across devices, you can create a manual save immediately prior to eating the third skull and unlock it on all your devices by simultaneously loading that save on all three devices and then eating the skull.