Leviathan

Isaac turns into a Lovecraftian horror, gains 2 black hearts, and gains flight after picking up three of these items.

Passive Collectibles
Note: Brimstone no longer counts towards the transformation as of Repentance.

Repentance only:Note: has a 1% chance to transform Isaac into Leviathan when he steps on the spikes created by this item.

Trivia

 * This transformation is based on the biblical creature of the same name.
 * This transformation was initially misspelled as "Levathian" in Afterbirth but has been fixed in Afterbirth+.