Binding of Isaac: All Transformations Guide & Required Items

Three items from the same theme picked up in a run, and Isaac changes body: transformations are among the most powerful mechanics in The Binding of Isaac: Rebirth, and among the least well explained by the game itself. No menu lists the item sets, no bar shows progress: without an outside reference, you trigger Guppy or Leviathan by accident, without knowing how to reproduce it.

This guide lists the 15 transformations available with Repentance+, the items that count toward each one, their exact effects, and the levers to trigger them on purpose. Since the game has no official French version, all item and transformation names are kept in English, exactly as they appear in-game.

Now is a good time to dive back in: buoyed by Repentance+'s online multiplayer and a big summer sale on Steam, Isaac hit record player peaks this summer 2026, more than ten years after its release. Many new players are discovering these mechanics for the first time.

How a transformation is triggered

The general rule: pick up three different items belonging to the same thematic set (flies, mushrooms, Mom's items…). Since Repentance, every item carries internal tags, and it's that tag (fly, mushroom, devil, angel, book…) that determines which set it belongs to. A few details that change everything:

• Active items count permanently: you can pick up Guppy's Head and then replace it with another active item, the progress remains locked in.
• Duplicates count since Repentance: two copies of the same item (obtained via Diplopia, for example) each advance the transformation, unlike in earlier versions of the game.
• Trinkets don't count, with two exceptions: Kid's Drawing for Guppy and Bob's Bladder for Bob.
• Pills only count for Adult and Stompy, never for the other sets.
• Starting items count: Judas starts with The Book of Belial, which is already a third of Bookworm.
• Watch out for rerolls: since Repentance, losing the required items (D4, D100, Sacrificial Altar…) removes the transformation. Notable exception: Tainted Isaac keeps his progress when he replaces an item in his limited inventory.
• Transformations stack without overwriting each other: an Isaac who is Guppy, Leviathan, and Seraphim all at once is entirely possible.

The summary table of the 15 transformations

The transformations that win a run

Guppy, still the benchmark

The oldest transformation in the game remains the strongest: permanent flight, plus a Blue Fly spawned two times out of three when a tear connects. On a build with decent fire rate, the screen fills up with allied flies that chew through everything, bosses included. Guppy items are mostly found in Devil Rooms and Red Chests, which makes it the natural reward for runs built around demonic pacts.

Two subtleties: since Guppy's Head and Guppy's Paw are active items, you only need to pick them up once and then switch back to your previous item to bank the progress without sacrificing anything. And since Repentance, if Dead Cat's nine lives run out, the item disappears from the inventory — which can cost you the transformation if you were sitting at exactly three items.

Leviathan and Seraphim, flight as a bonus from your pacts

These two twin transformations reward loyalty to a side. Three items from the devil tag — Brimstone, Pentagram, The Mark, The Pact, Abaddon, Spirit of the Night, Maw of the Void, The Nail, Sulfur, plus Eye of the Occult and Lord of the Pit since Repentance — trigger Leviathan: flight and two black hearts. Three items from the angel tag (The Halo, Holy Grail, Holy Mantle, Mitre, Rosary, Dead Dove, The Bible…) trigger Seraphim: flight and three soul hearts.

A lesser-known tip on the Seraphim side: each distinct copy of The Bible picked up counts separately. With Rosary, which injects The Bible into the item pools, it becomes realistic to complete the transformation using only bibles found along the way. Also worth noting, the familiar named Seraphim doesn't count toward the transformation of the same name — however, it does count for Conjoined.

Spun and Yes Mother?, raw power

Spun is the offensive transformation par excellence: +2 damage and +0.15 speed for three syringes (Roid Rage, Synthoil, The Virus, Speed Ball, Growth Hormones, Adrenaline, Euthanasia, Experimental Treatment), with a free pill thrown in. A flat damage bonus of this magnitude transforms any early run.

Yes Mother? summons a stationary Mom's Knife that trails behind Isaac and deals twice its tear damage on contact — a formidable melee tool against bosses. With nearly twenty Mom items in the pools (Mom's Contacts, Mom's Heels, Mom's Key, Mom's Bra, Mom's Bottle of Pills…), this is one of the most frequently triggered transformations in the game without even trying.

The secondary transformations

Beelzebub deserves a mention: permanent flight, and all small enemy flies converted into allied Blue Flies — swarm rooms in the Caves empty themselves out. Since Repentance, big flies (Boom Flies, Pooters, Suckers…) even become friendly and follow Isaac from floor to floor. First successful transformation? You also unlock the Angry Fly item and the Lord of the Flies achievement.

The rest is a case-by-case matter. Fun Guy (three mushrooms) and Adult (three Puberty pills) each grant a heart container. Bookworm gives roughly a 25% chance to fire a double tear, at full damage since Repentance and unaffected by Luck — excellent for big-tear builds. Bob leaves a poisonous trail dealing 20 damage per second and grants immunity to poison, with Ipecac counting toward the set. Conjoined adds two faces that shoot diagonally at the cost of a slight stat decrease: the list of eligible familiars has expanded significantly with Repentance (Incubus, Lil Brimstone, Ghost Baby…). Oh Crap restores half a red heart per pile of poop destroyed. Spider Baby provides a familiar that applies random status effects on contact. Stompy, finally, lets you crush rocks and obstacles by walking over them since Repentance — a real time-saver for exploring floors.

Super Bum is a unique case: no thematic set, but three specific familiars — Bum Friend, Dark Bum and Key Bum — that merge into a Super Bum which automatically collects coins, hearts, and keys in the room, for twice the rewards compared to the three separate beggars.

Forcing a transformation: what actually works

• Exploit the items: picking up an eligible item then picking the old one back up costs zero item slots and counts as one-third of the transformation. This applies to Guppy's Head, Guppy's Paw, The Bible, Jar of Flies, Mega Mush and others.
• Duplicate: Diplopia and other sources of duplicates count since Repentance. Two copies of the same Guppy item plus a third different one are enough.
• Learn the counter-intuitive tags: How to Jump is a book that counts for Bookworm, Ipecac counts for Bob, and Skatole counts for both Beelzebub and, since Repentance, for Oh Crap.
• Don't reroll a transformed build: D4, D100 or Sacrificial Altar can remove the set items along with the transformation.
• In internal duo (Jacob and Esau, Tainted Lazarus): only the character who picks up the item first banks the progress — each one must complete their own set.

Which transformation to aim for depending on your character

The right instinct isn't to chase a transformation, but to know which one is within reach. Azazel and The Lost already fly: for them, Guppy is only worth it for its Blue Flies — always excellent — while Leviathan or Seraphim lose half their appeal. Conversely, characters without red containers (???, The Lost, Keeper and their Tainted variants) get nothing from Oh Crap, Fun Guy or Adult: ignore these sets and save your resources for something else. For a Tainted Lost, only offensive power matters: Spun, Bookworm and Yes Mother? are the only options that truly change a run. And if you're playing a classic character focused on Devil Rooms, consider Leviathan your default plan: you'll pick up its items anyway, so you might as well know the threshold — the third pact isn't just an item, it's also flight and two free black hearts.