Transformations

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.

Requirements

 * Items only need to have been acquired once in order to count.
 * Duplicate items (like those acquired from ) do not count toward transformations when picked up. Only the first instance of an item Isaac owns will count.
 * Duplicate items are all counted.
 * Activated collectibles (e.g. ) can be picked up once and then swapped away with another without any loss to transformation progress.
 * Rerolling items with a dice room, the, or the will not remove progress towards the transformation.
 * The transformation will be lost if the items required are rerolled or otherwise lost, eg..
 * Familiars gained from or  do not count towards transformation progress.
 * Trinkets do not count toward progress for any transformation, with the sole exception of Kid's Drawing that counts for Guppy transformation.
 * Pills only count toward Adult and Stompy, they do not count toward progress for any other transformation.
 * Characters that start with an item that counts towards a transformation do count as one of the three items needed for a transformation, such as Judas starting with The Book of Belial.
 * When playing as or  only the character that picks up an activated item first gets the progress for related transformations, however both characters in the set may undergo the same transformation if they both collect the remaining required set items left in the item pool.
 * will not lose a transformation nor progress towards it if he replaces an item that counts towards that transformation.

Trivia

 * There are two transformations in the base game, nine more in the Afterbirth DLC, and four more in the Afterbirth † DLC.
 * 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.
 * The reason no new transformations have been added to Repentance is because Edmund and the dev team do not like them, as they view them as significant advantages gained only from being lucky enough to find specific items. This is why Guppy, the most powerful transformation, requires much more effort to get compared to a transformation like Spider Baby. If any new transformations were to be added, they would be purely visual.