The Binding of Terraria

You need both games: Terraria 1.4.5+ and The Binding of Isaac: Rebirth (Steam). Isaac's monsters, bosses, items, sounds and room art are read from your own copy of Isaac; nothing from either game is in this download. On your first Play the mashup unpacks Isaac's art with Isaac's own official ResourceExtractor (about a minute, once). With Afterbirth or later you get every boss and enemy; without it, the DLC bosses stay out of the pools, as in Isaac.

A trapdoor appears next to your Terraria spawn point. Stand on it and press Down to drop into a fresh Isaac run, played side-on with your own Terraria character and gear.

The run

• Eight floors like Isaac's: Basement I and II, Caves I and II, Depths I and II, Womb I and II. Every floor is a new Isaac-style map with a minimap, a treasure room and a boss room, in that chapter's own art. The boss's trapdoor takes you down to the next floor.
• Every floor rolls its boss the way Isaac does, from that chapter's pool with Isaac's own weights, never one you already met this run:
  • Basement: Monstro, The Duke of Flies, Gemini, the Gurglings
  • Caves: Gurdy, Peep, Chub, Gurdy Jr.
  • Depths: Monstro II, Gish, Loki, The Cage
  • Womb: Lokii, Blastocyst, Mama Gurdy
• Mom waits at the bottom of the Depths: her foot stomps down on you, her eye fires blood lasers out of the walls. It Lives hangs from the ceiling at the bottom of the Womb. Kill it to win the run and climb back out.
• 25 of Isaac's monsters with their own art, animations and sounds, on the floors they live on in Isaac: Gapers, Chargers, Globins that get back up, Boom Flies that explode, Hosts that hide under their skulls, Knights you can only hit from behind, Vis with blood lasers, Leeches, Suckers, Babies that teleport and more.
• Doors lock while enemies are alive and open when the room is clear. Walk through side doors, jump up through ceiling doors, hold Down on a floor door to drop to the room below.
• Isaac pickups: hearts heal you, soul hearts shield you, coins become Terraria coins, keys open treasure rooms (the first floor's is open), and bombs (G) blow up rocks and monsters. Bombs, keys and soul hearts carry over from floor to floor.

Items you keep 42 Isaac collectibles from Isaac's treasure and boss pools change your Terraria character for good: damage, attack speed, triple and quad shot, homing, piercing and spectral shots, more life, regeneration, damage resistance, poison, crits and more. Hover an item in your HUD to see exactly what it does in Terraria; right-click it to drop it for good.

Your world is never changed: the run borrows a patch of the world while you are in it and gives it back exactly, and world saves always write the untouched world. Dying or leaving the world ends the run; the items you took stay with you.

Single player. Multiplayer worlds don't get the trapdoor.

Credits

• Harmony by Andreas Pardeike (MIT), bundled.
• Terraria by Re-Logic. The Binding of Isaac: Rebirth by Edmund McMillen and Nicalis.
• Built with AI assistance (Claude Code with the universal-modder toolkit).