"The World You Only Knows" achievement itself should unlock automatically when you receive all other achievements.
As I write this in version 1.0.13 there seem to be a bug prevent me to get that, so if you have all other achievements without this final one, this guide may help.
We will install a older version of the game to get it.
Steam however will prevent we from installing an older version, so we need to use this tool SteamManifestPatcher[github.com] .
Download the SteamDepotDownpatcher.exe and run it while Steam is running. Success patch should look like this.
Enter to close it.
Download Old Version
Press (Windows key) + R or Start -> search Run.
Put "steam://open/console" -> enter to open Steam console.
Put "download_depot 1506280 1506281 5958058557287333319" to Steam console -> enter
The download will start, you won't see it in Steam download section, but you can see it's downloading in current network usage.
Wait until download complete:
Look the manifest should be "5958058557287333319"
Get The Achievement
Now copy all file from "(steam)\steamapps\content\app_1506280\depot_1506281" to "(steam)\steamapps\common\Erophone". Yes to overwrite all of them.
Run the game, now the game version at bottom right should be 1.0.12.
Play a level, you should receive the achievement at the end.