When obtained as any character other than Fiend, will periodically create fireball wisps that will orbit around the player, up to three.
Double-tapping a shoot button will cause an orbiting fireball to be launched in that direction as a fast projectile with weak homing, leaving a trail of flames and exploding on contact with an enemy or obstacle. The fireballs themselves deal 3× Isaac's damage on a direct hit.
The rate (per second) at which the fireballs are generated scales with Isaac's tears stat, with the number of seconds being based on the formula ((30 ÷ Tear Rate − 1) × 7) ÷ 30. The longest maximum delay is capped at 5 seconds at 1.34 tears per second.
The fireballs' explosion, and the flame trail created by the explosion deal independent damage based on whether they collide with an enemy or not:
Upon a direct hit with an enemy, the explosion deals a flat 2 damage, and the flames deal 1/4 of Isaac's damage.
Upon colliding with anything else, the explosions deals 6 damage, and the flames deal 1/12 of Isaac's damage.
The damage of the fireball upon directly hitting an enemy cannot go below 10.5, and the flame trail's damage cannot go below 0.9.
Fireball explosions cannot damage the environment (Rocks or doors), and do not harm Isaac.
Fireballs are not generated automatically, instead one will spawn when Fiend deals enough accumulated damage to enemies, requiring a base total of 50 damage, and increases by 10 for every next floor.
If a fireball launched by Fiend kills an enemy, it has a 50% chance to drop a Half Immoral Heart that flickers and disappears after 2 seconds.
Notes
Fireballs that are orbiting the player will not deal any contact damage to enemies.
The Range of the launched fireballs is infinite, and their Shot Speed is fixed and does not scale with the player's own Shot Speed.
Fiend: If Fiend gains a copy of the Pyromancy item, increases the number of fireballs that can orbit Fiend by 3. The behavior of Fiend's own innate fireballs is unchanged.
Abyss: Absorbing Pyromancy spawns 3 Abyss Locusts that look like fireballs. These locusts deal 33% damage.
The locusts spawned from Pyromancy has a low chance to instead appear as a burning skull with a tophat.
Video Showcase
Trivia
Pyromancy was unused in the initial The Binding of Isaac: Afterbirth+ release of the Fiend Folio MOD.
The Abyss Locust that appears as a burning skull with a tophat is a reference to the avatar of one of the Fiend Foliodevelopers, Erfly.
Fiend, will periodically create fireball wisps that will orbit around the player, up to three.
Rocks or doors), and do not harm Isaac.
Half Immoral Heart that flickers and disappears after 2 seconds.
Blue Fire Places and 
Purple Fire Places.
BFFS!: BFFS! and other familiar modifiers will have no effect on the orbiting or launched fireballs.
Sacrificial Altar: Orbiting fireballs can not be sacrificed.
Abyss: Absorbing Pyromancy spawns 3 Abyss Locusts that look like fireballs. These locusts deal 33% damage.
