The Binding of Isaac: Rebirth

You can only unlock this achievement if you have the Afterbirth + DLC

To unlock this achievement it requires you to defeat Hush, the boss of the Blue Womb, thus unlocking the new floor The Void.

You must beat Mom's Heart/It lives before the 30 minute mark which will leave a hole in the wall that leads to to the entrance to the Blue Womb

For tips for the boss, refer to the wiki here
https://bindingofisaacrebirth.gamepedia.com/Hush