Defeating him ends the run and gives you the final ending cutscene. Once you get to Delirium be warned hes the hardest boss in the game, he has massive health and constantly moves quickly and can spawn into any boss in the game. Make sure if your going to the Void you pick up a teleport pill or card as you can then find where Delirium is and teleport out to do the rest of the floor as the other 7 bosses give boss items. Also the floor has 8 bosses! and the floor is bigger than the biggest XL floor in the game! I have done a test run where I was fighting him with 1 damage and 0 tear delay, the fight seemed to go exactly the same as when I fought him with 999 damage and 0 tear delay. The Void does not follow normal floor rules in that each room is a random room from any floor in the game. This is only speculation, but it would appear that Hush's HP will scale off of the damage dealt to him. *50% after Mega Satan (If the portal appears, the run will not end instantly like it normally does) *5% after Mom (Only if Isaac has 10+ tear damage or 20+ items) 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

The binding of isaac rebirth hush