The End Achievement
Complete the story.
After you've restored the Bastion completely, you'll have to choose whether you want to restore the old world or stay. Choose the first one to restore the old world. This will only net you this achievement. You won't get The Beginning (Stay and embrace the new world). But since you need a second playthrough for Calamity Kid (Beating New Game +), just pick one choice the first time and the other one the second time around.