The Beginning Achievement
Complete the story, embracing the New World.
After you've restored the bastion completely, you'll have to choose whether you want to restore the old world or stay. Choose the latter one to embrace the New World. This will only net you this achievement. You won't get The End (Using the bastion to restore the old 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.