The Binding of Frisk

Undertale x The Binding of Isaac: Rebirth. You play Undertale as Frisk, and Isaac's basement comes up into the Underground.

Isaac is your SOUL

In every battle the red heart is Isaac's head. Hold Z during the enemy's turn and he fires tears up at the monsters. They pop Isaac bosses' shots and chip at any monster's HP, but the last blow is still yours to choose: FIGHT or MERCY.

One Isaac boss in each area

The first random encounter in each area is that area's Isaac boss, in a real Undertale battle with Isaac's own sprites, sounds and boss music:

• Ruins: Monstro. Blood-tear spit and jump stomps. ACT: Feed him.
• Snowdin: The Duke of Flies. A chasing swarm and a closing ring of flies. ACT: Buzz along.
• Waterfall: Gurdy. Wall sprays and tear rings. ACT: Smile.
• Hotland: Mom. Her foot stomps down and her eye weeps blood. ACT: Cry.

Spare them or beat them. Either way they drop an Isaac item.

Isaac's items

The Sad Onion, Spoon Bender, Cricket's Head, Lunch, The Inner Eye, Polyphemus, Brimstone and The Halo work as Undertale items. Use one to swallow it and upgrade Isaac's tears for the rest of your run: tears up, homing, damage up, max HP, triple shot, mega tears, a charged blood laser, all stats up. Every shop sells one too: Snowdin, Tem Shop, Gerson, Bratty and Catty, and MTT Resort.

Good to know

• Single player. It saves to its own folder, so your normal Undertale save is never touched.
• Needs Undertale (Steam, v1.08) and The Binding of Isaac: Rebirth (Steam).
• Nothing from either game is included. The first Play reads Isaac's bosses, items, music and text from your copy (about 2 to 3 minutes, once), and plays a patched copy of your Undertale. Neither game's folder is changed.

Credits: Undertale by Toby Fox. The Binding of Isaac: Rebirth by Edmund McMillen and Nicalis. Uses xdelta3 (GPL-2.0, source included). Patch built with UndertaleModTool.