Stage Additions
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.
Chapter 1
Basement
Basement, as the most classic and the easiest floor in the game, didn't get as much attention as some other floors. But that doesn't mean it didn't get plenty of additions as well!
Fiend Folio's Basement is themed around flies, poop, and Isaac himself. See brand-new foes like 
Bumbler and 
Dung, and watch out for 
Slim's fists and 
Slammers coming down from above!
Cellar
A familiar floor originating from Wrath of The Lamb, 
Cellar is bustling with more creepy crawlies than ever!
Careful around these new arachnid critters like 
Spooters and 
Sticky Sacks! Not only that, seems like some of 
The Haunt's ghostly relatives like 
Haunted and 
Shirk are lurking around! Spooky!
Honeyed Cellar
Cellar was always a spider-infested place. But what are those? Bees?
It seems like bees and wasps now call the Cellar home as well! Throughout your journey across this place, you may encounter some rooms whose walls are covered with honey and patches of honeycomb.
Bee-ware! These little insects like 
Hover and 
Zingling want only one thing, and it's to stab you with their stingers!
Burning Basement
Burning Basement is hotter than ever! With Fiend Folio, a new mechanic comes — Gunpowder!
Many enemies in Burning Basement leave flammable powder in a similar fashion to creep. The powder on its own is harmless, but as soon as it gets in contact with flames or an explosion, it gets really nasty!
You'll find a lot of new burning enemies as well, like 
Flamin and 
Woodburner! Try not to get singed!
Smoky Basement
Some rooms are almost completely burnt down and have been covered in soot! Be prepared to find some smokin' hot foes like 
Fumegeist and his 
Motes!
Chapter 1.5
Downpour
Downpour is now filled with more specters than ever before! Apparitions and Will-o'-the-wisps will assault Isaac as he tries to find that first Knife Piece...
Dross
Dross is a contender for one of the grossest floors you can visit! Wade through nasty sewer water as gaseous enemies and living poop assault all senses!
Peepee Dross
Didn't get enough sewage before? Dross is up to its knees in both fecal matter, as well as a strange, yellow substance which numerous foes will use to their advantage!
Chapter 2
Caves
The 
Caves are brimming with life! Prehistoric creatures, ancient Dwellers, malicious Mushrooms, batty Bats, and all other kinds of organisms and aberrations await.
Primordial Caves
Some rooms look more ancient than most. If you take a closer look, you can spot some prehistoric drawings on the rocks and walls have some fossils in them!
Catacombs
If you thought the 
Catacombs were boring, not anymore! This floor received a completely new theme! It now contains many cryptid-based and outright strange enemies like 
Mothman, 
Matte, and 
Sniffle.
Catacombs also brings back a few of your favorite spiders and even the buzzing bees! Some enemies seem to be oddly appetizing as well…
Flooded Caves
The 
Flooded Caves are even more flooded! Foes here aren't afraid to get a little wet, with some enemies like 
Madclaw, 
Fishface, and 
Frog diving beneath and out of the water to cause Isaac trouble!
Wading through these caves, you'll also find many enemies that fill the air with 
Bubbles, like 
Wetstone and 
Lily Pads that have trouble supporting Isaac's weight and sink when stepped off of! Better hope Isaac knows how to swim…
Chapter 2.5
Mines
Within the cavernous tunnels of the 
Mines, you will find a host of new foes — from the sizzling-hot 
Fuegos and 
Roasties and the volatile 
Blastcores to their even worse big brother, 
Rufus!
But what's this? Some of your opponents seem to have harnessed the power of technology...!?
Ashpit
With the constant smell of smoke permeating the parched, dusty air, this place feels like a wasteland… that is, when it isn't bustling with decrepit adversaries of all shapes and sizes! The 
Ashpit badlands are only for the brave of heart!
Chapter 3
Depths
The 
Depths are haunted! Face the most terrifying ghosts like 
Gnawful, 
Spook, or 
Mistmonger, fight against the 
Scythe Rider, and duel against the 
Squire while exploring this ghoulish place!
And tread lightly, some foes are truly frightening...
Necropolis
In 
Necropolis, skeletons rise from their graves! Are you brave enough to face monsters like the 
Possessed, 
Ribeye, or 
Baro in this city of the undead?
Dank Depths
Filthy, slimey, gooey, dark… many such words describe the 
Dank Depths and the new grimy goons like 
Gunk, 
Gis, and 
Gorger that stalk its sticky halls! Watch your step!
Pipe Depths
Some rooms appear to contain remains of old sewage systems, creating an industrial theme. Beware of the sludge-pumping pipes and those that dwell in them, like 
Grater and 
Clogmo!
Trashy Depths
Discarded items and trash litter the deepest pits of the Dank Depths. Some enemies such as 
Bunkter and 
Litter Bug have even made their home there!
Chapter 3.5
Mausoleum
A sacred place defiled, occult magicks and undead horrors have made their home here. Battle against fanatic officiants, possessed tomes, vengeful spirits, gargantuan husks, and all kinds of psychic adversaries as you explore the once holy halls of the 
Mausoleum!
Alt Mausoleum
Back by popular demand! Antibirth's Teleporter Mausoleum design makes a triumphant return as a possible backdrop for several Mausoleum rooms!
Gehenna
A terrifying house of torture, heresy, and entirely too much black leather, 
Gehenna is bursting at the seams with untold numbers of cultists, demons, and just-plain-nasties out to end you once and for all! Can you make it through these blood-stained halls and live to tell the tale?
Chapter 4
Womb
Return to the 
Womb, a place where unborn fetuses, sentient flesh, and other disgusting creatures dwell…
Utero
Utero is a truly bizarre and alien place where organs roam on their own! Brains and guts sliding about, 
Astroskull and 
Minimoons drifting aimlessly, 
Mobile Blood Cells on an endless march... And a heart's beat that echoes through the air...
Eyeball Utero
Some of the walls in Utero have eyes! Mother is watching!
Scarred Womb
The smell of blood hangs in the air… The 
Scarred Womb is definitely not a safe place! Do not stop or the twisted and terrifying abominations of this domain will eat you alive!
Chapter 4.5
Blue Womb
Blue Womb also got a grim-up. This can be especially well seen when using the 
Blue Key. Check out these fancy rocks!
Corpse
A place that was once alive now only harbors all that is revolting, 
Corpse is where the deceased and the rotting fester about...
Watch out for putrid foes such as the freaky 
Fingores or the withered 
Whales, as they're only the first few tribulations Isaac must survive in the Corpse before facing off against 
Mother!
Morbus
Has the Corpse not been vile enough for you? Fear not (or perhaps, DO fear), because Morbus makes a debut as an alternate subtheme for Corpse in Fiend Folio: Reheated!
Morbus is a necrotic, festering place, home to that which should have never lived... or, perhaps, continued to live.
Chapter 5
Sheol
Tread carefully within the realm of 
Satan himself, 
Sheol! Demons like 
Ms. Dominator and 
Psi Hunter are at their most powerful here and will not hesitate to kill on sight!
Psionic Sheol
A new species has invaded Sheol! 
Psion and their lackeys have come from beyond the Void to cause chaos!
Cathedral
Virtuous angels, hypnotic watchers, and all that is holy; these are the inhabitants of 
Cathedral. Prepare for a big fight if you're brave enough to enter their hallowed ground.
Chapter 6
Dark Room
The 
Dark Room is a desolate place where memories come to die. Prepare to meet monsters from all chapters, ready to put you six feet under!
The Chest
A place for everything that has been forgotten. Are you ready to fight your own nightmares? If yes, 
The Chest awaits with its lid open.
Endgame
The Void
Although 
The Void is a fusion of all chapters, it also got some touchups! Rocks and pits are totally randomized and do not care about the backdrop!
Home & Ascent
Your past foes have grown stronger! Make the climb back up 
Home, but watch out; horrific monsters such as the 
Reapers and 
Missing Links want to send you back down into your grave!
Special Rooms
Various Special Rooms in the game also got rehauled, giving them a completely new look!
Shop
Looks like 
Greed has made some renovations! 
Shops are now more shoppy!
Arcade
Arcades greet you with their new appearance! Gamble, play, and have fun!
Challenge Room
Challenge Rooms got a totally unique look! Feel like a champion and fight for your reward!
Secret Room
Secret Rooms are more ominous than ever! Who knows what you might find this time?
Crawl Space
Crawl Spaces are more exciting than ever! Prepare for crazy puzzles, platform jumping, and unique enemies, all in the fantastic coat of one of seven different backdrops!
Luscious Crawl Space
This version of the Crawl Space is overgrown with plants!
Ossuary Crawl Space
This version of the Crawl Space appears to be a mass grave. Bones and skulls are everywhere!
Pipe Crawl Space
This version of the Crawl Space seems to be a part of an abandoned sewage system.
Creature Crawlspace
This version of the Crawl Space appears to be the insides of some massive creature. Expect to see 
Onlookers and 
Spinning Blood Cells show up here!
Fortress Crawlspace
This version of the Crawl Space appears to be some kind of castle or dungeon.
Insulation Crawlspace
This version of the Crawl Space looks like the dingy spaces inside of the walls of a home.
Dice Room
In 
Dice Rooms, everything is a dice! Rocks, pots, and even dice are made of dice!
D12 Dice Room
Fiend Folio: Reheated also adds a new blue variant of Dice Room, whose effect activates the 
D12 effect in each room for the entire floor!
I AM ERROR Room
I AM ERROR rooms, as their name suggests, are full of errors. Rocks are glitched and out of bounds!
Library
Libraries are full of books; from learning 
How to Jump to 
The Bible, all books can be encountered in these musty rooms! Now go read a book.
Other Additions
Fiend Folio focuses on small things as well! Items like D12 are more enjoyable when playing with FF!
D12
Peepee Dross Alt Rocks
With Reheated, a new alternate backdrop in Dross, known as Peepee Dross, can appear with pee-filled 
Buckets instead of poop!
When destroyed, they drop similar things as those of regular poop-filled buckets in Dross, but spawn 
Striders and 
Small Leeches instead of 
Drips and the projectiles are yellow and leave harmful creep where they land.
Morbus Alt Rocks
With Reheated, Morbus rooms in Corpse have Teeth instead of the usual 
Polyps! These can hold some morbid surprises when cracked open!
When destroyed, a tooth in Morbus may spawn:
Nerve Ending, bearing a unique appearance.
Oralid
Bone Heart
Morbid Heart
Worm Friend
Chapter 6 Alt Rocks
Dark Room's gravestones and Chest's dolls can hide some spicy surprises! Watch out with those bombs!
When destroyed, a gravestone in Dark Room may spawn:
When destroyed, a doll in The Chest may spawn:
Bony
Black Bony
Crepitus
Sackboy
Cracker- Possessed
Naked Curdle
Neonate
Detached Nuchal- Bone Heart
Cursed Penny
Trivia
- The
Bomb Rock texture for the Primitive Caves appears to have a small fossil resembling the mini-boss 
Wrath. - The Large 2x2 sized rocks found in Dice Rooms and created by The D12 and
Eternal D12 appear as monuments to the 
D100item. - One of the
Fool's Gold Rock in Mausoleum resembles a golden statue prop found in Amnesia: The Dark Descent.
- Another Fool's Gold rock appears as the
Dark Arts item stabbed into the rock.
- Another Fool's Gold rock appears as the
- The pillar textures in the Arcade appear as broken down game cabinets for Super Meat Boy and The End is Nigh, two games also made by Edmund McMillen.
- The texture of the Library's
Super Special Rock after its been damaged shows a hammer-shaped hole in the pages, referencing The Shawshank Redemption. - Some Fool's Gold rocks in Trash Depths reference Bitcoin and a portrait of a Bored Ape, an infamous NFT.
- Rocks in the Gehenna floor sometimes comprise of various torture devices; these include:
- Wooden horse.
- Judas cradle.
- Stretcher table.
- Iron Maiden.
- Pillory.
Marked Skulls in most rock sprite sheets (besides Depths, Necropolis and Dank Depths) have special sprites and can be found in very rare rooms. Furthermore, some of these sprites are easter eggs in reference to specific developers of Fiend Folio. A full list and gallery can be found here.- Using
XVI - The Tower? has the rocks use their un-modded textures, but re-entering the room fixes this.