The Void

The Void is a chapter added in Afterbirth+. It is unlocked by defeating Hush. The Void is one of five final levels in The Binding of Isaac, alongside, the ,  and.

Each room in this chapter takes the form of a different chapter, ranging from to  and, including variant chapters like  and the. The Void is extremely large compared to other floors, and it is the only floor in the game (not counting XL floors) to contain multiple boss rooms. Most of them contain random bosses from other floors, including some of the final bosses, such as It Lives, Satan, Isaac, etc. One of them, however, contains the main boss of the floor, which is always Delirium.

Access
The Void is unlocked by defeating Hush. After defeating Hush at least once, a new exit will appear in the upper right-hand corner of the 2x2 boss room in which Hush was fought. The new room contains nothing except for a Portal. It does not spawn enemies and serves as the entrance to The Void (the animation shows it trying to suck Isaac in but it's only visual).

After unlocking The Void, the portal can then appear after every major boss starting with Mom, even if Hush was not defeated in that run.

The deeper the boss, the higher the spawning chance:
 * 5% after Mom (Only if Isaac has 10+ tear damage or 20+ items)
 * 10% after Mom's Heart/It Lives
 * 15% after Isaac or Satan
 * 20% after ??? or The Lamb
 * 50% after Mega Satan (If the portal appears, the run will not end instantly like it normally does)
 * Defeating Hush will guarantee entrance to The Void.

Bosses
This floor contains multiple boss rooms, only one of which will contain Delirium. The other boss rooms contain previous bosses, including the other final bosses, except Mega Satan, Hush, Mother, Dogma, the Ultra Harbingers, and The Beast.

Locating Delirium
Main article: The Void/Locating Delirium
 * To locate boss rooms which will most likely contain Delirium, Isaac must look at the boss rooms on the map:
 * Try to find a boss room that would not be connected to other rooms if it was a 2x2 room. If it would connect, the room won't contain Delirium.
 * Remember that the boundaries of the map are at a maximum of 13x13 squares. If a 2x2 boss room would extend past those boundaries, that room cannot contain Delirium.
 * By knowing which boss rooms cannot contain Delirium, Isaac can safely enter certain boss rooms to gather items before fighting Delirium, or not enter these boss rooms knowing that Delirium is not there.

Trivia

 * It is implied that the different forms (floors from chapters 1 to 6) of this floor are just Isaac's hallucinations, as the Void's icon on the progress bar (after advancing to the next floor) is a red vortex, and the "floor" graphic (below the player character and bosses in "VS" screens) is a red vortex as well.
 * On Isaac's last will, The Void is referred to as "some dying memory".
 * Occasionally, there will be a static distortion and the background and objects of the room may change to those of other chapters.
 * Mom's Heart can still spawn as a boss, even after being replaced by It Lives in Chapter 4.
 * The Void is the only floor that can be cleared indefinitely with (or Teleport 2.0 plus The Battery or a guaranteed Little Battery drop).
 * The chances of champion enemies spawning in The Void is much higher than in any other floor, and they will never drop any of their assigned drops.
 * The Void can be entered in Greed Mode by entering "stage 8" in the Debug Console. The rooms have random backdrops and the music is the same as The Void in regular mode, but the boss is Ultra Greed and will drop the Final Boss Chest. Entering the chest will play Ending 20 instead of Ending 18 or 19.
 * If Mr. Fred appears as a boss, his room will be displayed on the map as a 1x1 room.
 * The starting room and the boss rooms of The Void do not have the visual effects of the environment they're based on (i.e. a boss room with a Womb layout will not have the red static of ).
 * Also, Caves-based rooms do not have the visual effects of.
 * If a chest spawns in a room with the Chest (Floor) or Dark Room layout, it will always contain an item.

Music
The music that plays in The Void is titled Terminus.
 * The music appears to be a remix of boss themes, Genesis 22:10 (title screen),, and themes.

Binding of Isaac: Afterbirth+ OST