The Void portal has the percent chance of appearing on the following floors once you kill a boss. Defeat the hush again - This will unlock the void portal automatically To get to the void you need to do one of two thnigs
Once you defeat Hush you unlock the void but you dont get to go in to the Void on that run you have to do it again. You do get 2 Choice treasure rooms and a shop on the hush floor but make sure you have high damage before going in. You need a good combo to get through it as he has an absolute ton of health. To unlock the Hush you need to defeat Moms heart in under 30 minutes that will unlock a blue door taking you to the hush floor. To unlock the void you need to beat Hush.
So this is is a 3 part achievement that can be done in two runs
