Lamb of Isaac

A new Basement door waits beside Darkwood in your cult's crusade room. Behind it, The Binding of Isaac's own monsters, items, music and friends, all read live from your installed copy of Isaac.

What you get

• The Basement crusade. Floor 1 is Isaac's Basement; everything after it is the Caves, with Isaac's real music playing (your Isaac plays it in the background).
• 25 Isaac items, including The Sad Onion, Cricket's Head, Magic Mushroom, Sacred Heart, Polyphemus, The Halo and Raw Liver. They drop after cleared fights and from bosses, with Isaac's own pickup banner. DMG up multiplies weapon and curse damage, tears up speeds up your swings, speed up makes you faster, and HP and soul-heart items add red and blue hearts. Items last until the crusade ends.
• Isaac bosses with their real sprites and moves.
  • Monstro guards the Basement's exit: he hops, spits blood-tear sprays and leaps off screen to land in a ring of tears.
  • Larry Jr. crawls at right angles dropping poop, and every segment can be hit.
  • Gurdy spits sprays and rings and summons Pooters.
  • Larry Jr. and Gurdy replace Darkwood's mini-boss on the final floor.
• Rescue Isaac, Magdalene, Cain and Judas. Each boss leaves one waiting. Free them and they join your cult as followers who keep their Isaac look.

Solo. A Basement crusade never advances your Darkwood progress.

Needs Cult of the Lamb, and The Binding of Isaac: Rebirth with Repentance or Repentance+ (Isaac mods need it), both through Steam on this PC.

How it works. The first start takes about a minute longer: the mod opens your Isaac straight into its first room, and a tiny Isaac mod draws Isaac's sprites so Cult of the Lamb can copy them into a local cache. After that Isaac waits in the background and plays its music and sounds on cue, and it closes when you quit. Isaac never leaves its first room, so your Isaac saves and unlocks are untouched. Nothing from Isaac is in the download, and Isaac's packed data files are never opened.

Credits: The Binding of Isaac by Edmund McMillen and Nicalis; Cult of the Lamb by Massive Monster and Devolver Digital. Built with AI assistance (Claude).