The Fiend Folio is an unlockable active item in Fiend Folio, a mod for The Binding of Isaac.

Summons one of 3 Fiend Folio bosses to fight for Isaac for a short amount of time.

The same boss cannot be summoned 2 times in a row.

Battie Charges down from the top of the screen homing slightly towards enemies (or Isaac if there are no enemies in the room) and wrapping vertically around the room 3-5 times, while leaving a trail of projectiles that float in place for a short time.

After charging across the screen, Battie finishes off with a slam onto remaining enemies, that creates a circular wave of rocks, then proceeds to fly away.

The slam, charge, and rock waves deal 10 damage per tick, while projectiles deal 3.5 damage.

Using the item against the actual Battie boss will always spawn Battie to fight against her. The final slam attack will instantly kill the boss and spawn Baby Badge.

Monsoon Slams down on the nearest enemy from above, splashing 16 water tears around him that each deal 3.5 damage. Right afterwards, he spits out a large cluster of 16 projectiles that deal 3.5 damage towards nearby enemies before ending the attack by splitting itself into a friendly charging Dribble, that sticks with Isaac until it dies.

The initial slam deals 100 damage. Monsoon deals 2 contact damage per tick.

Technopin Burrows out of the ground from a random location before quickly aiming and firing a laser towards the closest enemy that bounces off of walls, and deals 25 damage. This attack is repeated 3 more times.

Technopin deals 2 contact damage per tick.

Pollution Charges from the left or right side of the screen whilst leaving behind a trail of fire, then pauses in the middle of the room to perform one of 3 attacks chosen in a random order:
• Spit out 6 friendly Blots.
• Swipe with his bike, releasing an arc of 6 flames towards the nearest enemy.
• Spins his bike, releasing 3 spinning lines of blood shots around him.

Buster Charges from the left or right side of the screen 3 times, whilst leaving a trail of fire and spawning several friendly Commisions. After the 3rd charge, stops in place and fires 2 lobbed projectiles that create rings of 6 flames, aimed at the nearest Commisions around him.

Slammers Spawns a total of 8 different variants of Slammers, one after another, finishing up with a Thwammy.

Get all completion marks as Fiend on Hard Mode.

This item contributes towards the Bookworm transformation. Collecting 3 items from this set will make Isaac fire an extra tear 25% of the time.

The Fiend Folio can be found in the following item pools:

The following tags are applied to this item:

The following special synergies and interactions for The Fiend Folio have been documented.

* Note that not all possible synergies and interactions are listed here.

• Check out the item description on the mod's external Wiki.
• Download Fiend Folio from the Steam Workshop.