While the majority of content in Fiend Folio only adds new features to the game without modifying some of the existing gameplay mechanics of The Binding of Isaac: Repentance, there is a still a large handful of exceptions that expand some existing elements, and drastically change the feel of the game.
The stages are more alive than ever! From discovering prehistoric caves to exploring abandoned sewer plants, Fiend Folio adds new backdrops and changes to each floor to solidify their theme and feeling.
- Main Article: Stage Additions
Fiend Folio: Reheated also added new pill colors to memorize! These have a chance of appearing in place of the regular vanilla pill colors encountered.
Note: These new pill colors are purely cosmetic; they do not correlate to the types of possible pill effects, nor number of pill effects that appear in the pill pool, which will always remain at 13 per run.
For the new pills and their effects added in Fiend Folio, see Pills.
Vanilla Enemy Changes
Besides adding new enemies, Fiend Folio also modifies and improves a few existing ones. These modifications can be toggled in the Dead Sea Scrolls menu of the MOD.
Blue Conjoined Fatty
Dank Depths Host.
The Siren's skull Host.
Lil Portals are now able to spawn some of the following insects added in Fiend Folio:
Lil Portals will also have a unique skin if encountered in Gehenna, giving them a red appearance.
Several of Fiend Folio's foes, along with a plethora of vanilla enemies, have received a special "waiting" variant.
Waiting enemies will not spawn immediately when Isaac enters the room, but only after all other enemies have been defeated. Each enemy has their own way of appearing in the battlefield, such as burrowing enemies appearing from beneath Isaac's feet, leaping enemies falling from off-screen, or spider enemies rappelling down from the ceiling with spider silk!
- If Isaac is standing in the same spot where an enemy would appear, it will wait until Isaac moves out of the way before doing so.
The following bosses in Fiend Folio have been added to Boss Rush:
The following bosses from the vanilla game have been added to Boss Rush:
- Baby Plum
- Rag Mega
- Big Horn
- Sisters Vis
- The Pile
- Reap Creep
- Daddy Long Legs
The following bosses Delirium can transform into are:
- In the files, there is a Delirium skin for Kingpin, though it seems to be unused.