Upon using any active item, Isaac sends several firework rockets that ignores walls and obstacles, and will home in on enemies and explode on contact with them.
The damage of the fireworks' blasts deals 2× Isaac's damage (with a minimum damage cap of 7) and inherit his tear and bomb effects.
The strength of the fireworks can also have additional modifiers based on the active item used.
Firework explosions can destroy the environment such as rocks and closed doors, but will not deal damage to Isaac.
The amount of firework rockets launched depends on the active item used, and the charge capacity it has.
Notes
If the player has any firework rockets queued, Isaac will send out one firework rocket in a random direction every 4 frames (roughly 7.5 rockets every second), until there are no rockets left in the queue.
Firework rockets in the queue will continue launching after transitioning between rooms, and even when entering a new floor.
Firework rockets will automatically explode after 5 seconds if they haven't hit a target.
"Throwable" or "shootable" active items that are held above Isaac's head, like The Candle, will not create fireworks, even when the charge is consumed.
Rocket Calculation
For any regular items with room-based recharge, the base formula for the amount of base fireworks sent is equal to: (chargesInItem × numberOfExcelsior × 2) + 1
The variable numberOfExcelsior is the amount of copies of the Excelsior item is in Isaac's inventory.
Any items marked as "special" for how they charge, such as Berserk!, will instead fire 8 base fireworks.
Any items with charge based on time, such as Brown Nugget, will instead follow a formula equal to: (numberOfFramesNeededToCharge / 45) × numberOfExcelsior
This will also make the rockets deal 33% less damage. This damage reduction ignores the minimum damage cap of the rockets.
After the amount of base fireworks has been calculated, on the next available frame, a set of additional calculations will be done:
If the player did not lose any active charge, the fireworks will be set to 0, and the item will not activate.
+8 fireworks for any Broken Hearts added to Isaac's health bar by using the item.
+20 fireworks and 6× firework damage if the item was a one-time use.
+33% to the total fireworks if the Excelsior soundtrack mod is installed and enabled.
Interactions
Bomb Effects: Firework rocket blasts will have synergy with some items that affect Isaac's bombs:
Bob's Curse: Fireworks leave behind small lingering gas clouds when they explode that poison enemies and deal 5 damage per tick.
Bob's Bladder: Adds a luck-affected chance for fireworks to leave behind a tiny pool of green creep when they explode.
Hot Bombs / Fire Mind: Adds a luck-affected chance for fireworks to burn enemies hit, and leave behind a small extinguishable flame when they explode.
Sad Bombs: Adds a luck-affected chance for fireworks to create a ring of 10 tears when they explode. The tears deal (25 + Isaac's Damage) × 0.5 damage and are affected by any tear modifiers Isaac has on his tears.
Bomber Boy: Adds a luck-affected chance for fireworks to create small cross-shaped explosions on detonation.
Butt Bombs: Each firework explosion clouds up the screen on detonation, briefly confusing and dealing 5 damage to all enemies in the room.
Sticky Bombs: Fireworks will create a small puddle of white creep that slow enemies when they explode. Fireworks will not stick to enemies.
Scatter Bombs: Fireworks create 4-5 tiny bombs when they explode. These tiny bombs deal Isaac's damage when they explode, but can harm the player.
Glitter Bombs: Fireworks will have a chance to spawn a random pickup when they explode, and has a 15% chance to charm random enemies in the room. The chance to spawn pickups follows the same diminishing returns as regular Glitter Bombs explosions, with the chance being reduced by 1% for the current floor each time a pickup is spawned.
Nancy Bombs: Firework explosions will have random bomb effects, with the same possible effects as regular Nancy Bombs explosions.
Brimstone Bombs: Adds a luck-affected chance for fireworks to release 4 thin Brimstone beams in the cardinal directions when they explode. Brimstone beams will deal 4 damage per tick.
The Common Cold / Scorpio / Serpent's Kiss: Adds a luck-based chance for Fireworks to poison enemies on contact. Fireworks will also leave behind a small poison cloud when they explode.
Technology: Fireworks will connect a laser to Isaac's position when they explode.
Brimstone: Fireworks will release 4 Brimstone beams in the cardinal directions when they explode. These Brimstone beams are identical to Isaac's and will inherit any synergies/tear effects.
Godhead: Fireworks create a small damaging Godhead aura at the location they explode, the aura lingers for a moment before disappearing.
Polyphemus: Each firework will leave behind a lit bomb when they explode, which detonates shortly thereafter. These bombs will also inherit any bomb-altering item effects Isaac has.
The Parasite: Each firework will leave behind 2 lit bombs stacked together when they explode. These bombs only deal 40 damage each.
Cricket's Body: Each firework will split into 4 tiny lit bombs spread out when they explode. These bombs only deal 35 damage each.
Trisagion: Fireworks send out a ring of 8 Trisagion beams when they explode. These beams will not inherit any tear effects.
Tech X: Fireworks create a small laser ring at the location they explode.
Mysterious Liquid: Each firework's explosion will leave behind a medium sized pool of green creep. Applies to additional explosions such as from Bomber Boy. Any additional bombs spawned such as from Polyphemus will create large pools of green creep.
Fire Mind: Adds a luck-based chance for Fireworks to burn enemies on contact. Fireworks will also leave behind a small fire when they explode.
Parasitoid: Adds a luck-based chance to create a splatter of small pools of long-lasting white creep when Fireworks explode. Does not spawn any Blue Spiders or Blue Flies.
Lachryphagy: Fireworks will be considered tears and join up with other existing tears. Fireworks cannot combine with other fireworks.
Multidimensional Baby / Multi-euclidean: Fireworks can be duplicated when passing through Multidimensional Baby and enemies afflicted with multi-euclidean.
Angelic Prism: Fireworks will be duplicated if they pass through the prism.
Abyss: Absorbing Excelsior spawns a pink sparkling star-shaped Abyss Locust. Currently does not have any unique functions and acts like a default Abyss Locust.
'M: Excelsior will create 20 additional rockets that deal 6× more damage, when the currently held active item becomes rerolled by 'M after being used.
Everything Jar: The amount of rockets produced after using Everything Jar will always be the equivalent to that of an item with a maximum charge capacity of 12, regardless of the amount of charge in the item.
Angelic Lyre: Blacklisted by Excelsior — will not create firework rockets when this item is used.
Video Showcase
Trivia
Excelsior is a reference to Excelsior, an OST made by Fiend Foliodeveloper RENREN, composer of Fiend Folio's music. Excelsior is also an honorary sister MOD to Fiend Folio that replaces all the soundtracks in The Binding of Isaac. (mod link)
The item's costume resembles the cultists found on the thumbnail art and album art for the mod and album.
The item's function is a nod to the release trailer for Excelsior, a video of fireworks being set off with text displayed over it announcing the mod.
The initial teaser sneak-peak for this item was posted publicly on "EXCELSIORWEEN", the 1st anniversary for Excelsior's release.
The Candle, will not create fireworks, even when the charge is consumed.
Berserk!, will instead fire 8 base fireworks.
Brown Nugget, will instead follow a formula equal to: 
Broken Hearts added to Isaac's health bar by using the item.
Bob's Curse: Fireworks leave behind small lingering gas clouds when they explode that poison enemies and deal 5 damage per tick.
Bob's Bladder: Adds a luck-affected chance for fireworks to leave behind a tiny pool of green creep when they explode.
Hot Bombs / 
Fire Mind: Adds a luck-affected chance for fireworks to burn enemies hit, and leave behind a small extinguishable flame when they explode.
Sad Bombs: Adds a luck-affected chance for fireworks to create a ring of 10 tears when they explode. The tears deal 
Bomber Boy: Adds a luck-affected chance for fireworks to create small cross-shaped explosions on detonation.
Butt Bombs: Each firework explosion clouds up the screen on detonation, briefly confusing and dealing 5 damage to all enemies in the room.
Sticky Bombs: Fireworks will create a small puddle of white creep that slow enemies when they explode. Fireworks will not stick to enemies.
Scatter Bombs: Fireworks create 4-5 tiny bombs when they explode. These tiny bombs deal Isaac's damage when they explode, but can harm the player.
Glitter Bombs: Fireworks will have a chance to spawn a random pickup when they explode, and has a 15% chance to charm random enemies in the room. The chance to spawn pickups follows the same diminishing returns as regular Glitter Bombs explosions, with the chance being reduced by 1% for the current floor each time a pickup is spawned.
Nancy Bombs: Firework explosions will have random bomb effects, with the same possible effects as regular Nancy Bombs explosions.
Brimstone Bombs: Adds a luck-affected chance for fireworks to release 4 thin 
Brimstone beams in the cardinal directions when they explode. Brimstone beams will deal 4 damage per tick.
Mr. Mega / 
Bobby-Bomb / 
Fast Bombs / 
Nugget Bombs / 
Bridge Bombs / 
Musca / 
Slippy's Guts: No effect.
Ball of Tar, 
The Mulligan, 
Head of the Keeper, 
Holy Light, 
Compound Fracture etc.) will be applied to the fireworks as if they were tears.
Sinus Infection, 
God's Flesh, 
Explosivo, 
Jacob's Ladder. These items will have no effect.
The Common Cold / 
Scorpio / 
Serpent's Kiss: Adds a luck-based chance for Fireworks to poison enemies on contact. Fireworks will also leave behind a small poison cloud when they explode.
Technology: Fireworks will connect a laser to Isaac's position when they explode.
Godhead: Fireworks create a small damaging Godhead aura at the location they explode, the aura lingers for a moment before disappearing.
Polyphemus: Each firework will leave behind a lit bomb when they explode, which detonates shortly thereafter. These bombs will also inherit any bomb-altering item effects Isaac has.
The Parasite: Each firework will leave behind 2 lit bombs stacked together when they explode. These bombs only deal 40 damage each.
Cricket's Body: Each firework will split into 4 tiny lit bombs spread out when they explode. These bombs only deal 35 damage each.
Trisagion: Fireworks send out a ring of 8 Trisagion beams when they explode. These beams will not inherit any tear effects.
Tech X: Fireworks create a small laser ring at the location they explode.
Mysterious Liquid: Each firework's explosion will leave behind a medium sized pool of green creep. Applies to additional explosions such as from Bomber Boy. Any additional bombs spawned such as from Polyphemus will create large pools of green creep.
Parasitoid: Adds a luck-based chance to create a splatter of small pools of long-lasting white creep when Fireworks explode. Does not spawn any Blue Spiders or Blue Flies.
Lachryphagy: Fireworks will be considered tears and join up with other existing tears. Fireworks cannot combine with other fireworks.
Multidimensional Baby / Multi-euclidean: Fireworks can be duplicated when passing through Multidimensional Baby and enemies afflicted with multi-euclidean.
Angelic Prism: Fireworks will be duplicated if they pass through the prism.
Abyss: Absorbing Excelsior spawns a pink sparkling star-shaped Abyss Locust. Currently does not have any unique functions and acts like a default Abyss Locust.
'M: Excelsior will create 20 additional rockets that deal 6× more damage, when the currently held active item becomes rerolled by 'M after being used.
Everything Jar: The amount of rockets produced after using Everything Jar will always be the equivalent to that of an item with a maximum charge capacity of 12, regardless of the amount of charge in the item.
Angelic Lyre: Blacklisted by Excelsior — will not create firework rockets when this item is used.
