To get this achievement you have to break the unicorn. The unicorn will appear in Northeastern Mexico (there are 3 horses on the map where it appears) after you have completed all the other 4 horses of the apocalypse challenges (by breaking each of the four horses and then killing one of each type of undead with the horse Death) and you have completed the Undead Hunter challenges. The unicorn is easily identifiable by the trail of rainbows it leaves while running. Once you break it, the achievement will pop. Good luck. **Note - I killed one of the four horses (pestilence I think), and even after I got Death for completing the undead campaign, I was not able to complete the last challenge. I had to find and break pestilence, so be sure to break all the horses as you find them.