Transformations alter Isaac's appearance and grant him special abilities.
A transformation occurs when obtaining at least three different items from a particular set of items.
These collectibles always share a particular theme that corresponds with the transformation.
Items only need to have been acquired once in order to count. Keeping them is not required.
Duplicate items (like those acquired from Diplopia) do not count toward transformations when picked up. Only the first instance of an item Isaac owns will count.
Activated collectibles (e.g. Guppy's Head) can be picked up once and then swapped away with another without any loss to transformation progress.
Rerolling items with a dice room, the D4, or the D100 will not remove progress towards the transformation.
Since none of the Afterbirth transformations unlock an achievement or item, attaining them is not required for 100% completion of the game in terms of achievements. However, this is only true if you unlocked Mom's Contacts on Rebirth, as the Yes Mother? transformation will happen once you collect three Mom items on Afterbirth and Afterbirth †.
Many of the Afterbirth transformations override the charge animations of items such as Monstro's Lung and Brimstone, potentially making them more difficult to use.
There is now an option to show a charge bar that floats above Isaac.
In the original The Binding of Isaac flash game, the transformation into Guppy was the only one available, making it chronologically the first.
The Eternal Edition of the original The Binding of Isaac game added another transformation that turned Isaac into an angel after gaining 7 heart containers from eternal hearts.
Rebirth introduced Beelzebub as the chronologically second transformation.
The original name for the Beelzebub transformation was Lord of the Flies.
Super Bum
Bookworm
The Binding of Isaac: Rebirth
