This is attainable in Goat Ville.
Starting from the spawn point with the house behind you, head to the far right corner of the map. As you walk along the path, the screen will turn red and you will see a sacrificial circle with torches around it. For this achievement, you need to bring five animals or humans into the circle. With the Goat Queen mutator turned on, use your special ability (
) and summon five goats inside the circle. With five total objects in the circle, a ritual will begin (the objects will morph together and go haywire) and the message "BLOOD FOR THE BLOOD GOAT" will appear on the screen. This will change you into the Devil Goat, unlocking the achievement.