Upon clearing a room, 1-3 Fairy Fly familiars will fly in from off-screen that can come in three different types: little, round, and big. Fairy Flies deal contact damage to enemies, scaling with Golem's damage, and eventually dying after some time.
Little Fairy Flies are the most common and will chase after enemies, dealing 10% of Golem's damage per tick and occasionally shoot tears that deal half of Golem's damage. Occasionally, they will roll for a 20% chance to charge directly into an enemy to deal double Golem's damage, dying afterwards. This chance increases by an additional 20% for each living Little Fairy Fly after every cleared room.
Round Fairy Flies appear uncommonly and will bounce around the room, homing in towards the closest enemy to deal 25% Golem's damage per tick and swelling up while doing so. After swelling up enough they will burst, creating a ring of 8 tears.
Big Fairy Flies are the rarest, with only 1 appearing at a time, and hover slowly towards enemies, dealing 10% Golem's damage per tick. Shortly after they begin to chase an enemy, they will fly high into the air before slamming down, releasing a ring of 12 tears and creating a Giga Bomb explosion that deals 100 damage, creates a cluster of rock projectiles that deal 15 damage and destroys tiles in a diamond shape. This explosion can harm Golem if he is too close.
Clearing a Boss Room increases the chance for a Big Fairy Fly to appear to 40%, while clearing a Mini-Boss Room increases the chance to 20%.
Notes
The chance for each type of Fairy Fly to appear scales with Golem's luck and Trinket Multipliers, with larger types of Fairy Flies having a higher chance to appear as either Golem's luck or the trinket multiplier of Lost Artifact increases.
In addition to inheriting Golem's damage, tears shot by Fairy Flies will inherit most of Golem's tear effects, tear modifiers and his shot speed. However, they are unaffected by Golem's tear rate.
Each type of Fairy Fly can come in slightly different sizes and hues. This has no effect or correlation on the damage they deal or their behavior.
If Lost Artifact is dropped, all Fairy Flies currently in the room will orbit around it in a staccato pattern, changing between different orbit distances intermittently.
Hive Mind: All Fairy Flies will double in size, and their attacks deal double their damage.
Brimstone: All Fairy Flies will shoot thin, rainbow-colored Brimstone beams instead of tears.
Technology: All Fairy Flies will fire rainbow-colored Technology lasers instead of tears.
Tech X: All Fairy Flies will launch small Tech X rings instead of tears.
Tech X + Brimstone / Technology: Tech X rings shot by Fairy Flies will become small circular Brimstone rings.
20/20 / The Inner Eye / Mutant Spider / The Wiz: Little Fairy Flies will be affected by multi-shot item effects, making them shoot multiple tears at once according to the amount of multi-shot.
Spirit Sword / Mom's Knife: All Fairy Flies will hold a small sword or a knife respectively, and point it towards enemies.
Trinket Multipliers: Affects the number of Fairy Flies that can appear after clearing a room, and the chance for larger types of Fairy Flies to appear.
Lost Artifact was designed by one of the winners of the Stronger Than Golem challenge, Alter.
If Show Room Names is enabled in the MOD's Dead Sea Scrolls menu, each Fairy Fly will have a randomly-generated name that can be viewed while holding the "expand map" key. The length of their name scales with the size of the Fairy Fly, and is made up of random strings of names and titles.
The Big Fairy Flies have an additional 0.1% chance to include "Featuring Dante from the Devil May Cry Series" in their name.
In the Fiend Folio MOD's files, hidden in the folder containing the spritesheets for Fairy Flies, there is a text document titled lore.txt that contains the following message:
So there is the tale of the mama fairy, mama fairy has a lot of baby fairies, they like copying other animals to play, their best friend was the pink cat, they copied the games of the pink cat and they all followed the pink cat. The pink cat had a big pink heart, a nice blue ribbon and a great yellow star.
One day-
Isaac remembered this fairy tale, in the basement
In the corner of the spritesheet of the Big Fairy Flies is a low-opacity message reading “ate meat”, along with a pleading emoji, which is an inside joke among Alter’s friend group.
Giga Bomb explosion that deals 100 damage, creates a cluster of rock projectiles that deal 15 damage and destroys tiles in a diamond shape. This explosion can harm Golem if he is too close.
Boss Room increases the chance for a Big Fairy Fly to appear to 40%, while clearing a 
Mini-Boss Room increases the chance to 20%.
Trinket Multipliers, with larger types of Fairy Flies having a higher chance to appear as either Golem's luck or the trinket multiplier of Lost Artifact increases.
BFFS!: Fairy Flies are not affected by BFFS!.
Hive Mind: All Fairy Flies will double in size, and their attacks deal double their damage.
Brimstone: All Fairy Flies will shoot thin, rainbow-colored Brimstone beams instead of tears.
Technology: All Fairy Flies will fire rainbow-colored Technology lasers instead of tears.
Tech X: All Fairy Flies will launch small Tech X rings instead of tears.
20/20 / 
The Inner Eye / 
Mutant Spider / 
The Wiz: Little Fairy Flies will be affected by multi-shot item effects, making them shoot multiple tears at once according to the amount of multi-shot.
Spirit Sword / 
Mom's Knife: All Fairy Flies will hold a small sword or a knife respectively, and point it towards enemies.
Azurite Spindown: Using Azurite Spindown on Lost Artifact rerolls it into 
Hiddenite.
