Replaces Isaac's tears with a sword he can swing in front of him.
The swing does 3x Isaac's damage +3.5 and deals knockback to both enemies and Isaac if it hits.
The sword can be swung as rapidly as the fire button can be tapped
The sword swing can collect pickups.
If Isaac's total health is equal to or greater than his number of Red Heart containers (same rule as regular Challenge Rooms), the sword fires a projectile when swung.
The projectile deals damage equal to Isaac's damage +2, and has a cooldown equal to 2x Isaac's normal tear rate.
Holding an attack button charges a spin attack, ready when the sword flashes, which damages all enemies around Isaac. It also deflects enemy projectiles and launches a projectile regardless of Isaac's current health.
The circular swing deals 8x Isaac's damage +10, and the projectile created from it deals 4x Isaac's damage +4.
The charge time is based on Isaac's tears stat, at ~4x Isaac's normal tear rate.
While charging, the sword will damage the first enemy that contacts it. This will cancel the spin attack.
Spirit Sword has the highest priority of all tear modifiers in the game, over Epic Fetus and Mom's Knife.
Unlike most charge attacks, the direction of the attack can't be changed by pressing and holding a different attack button then releasing the old one. Instead, it will stay pointing the same direction until all attack buttons are released, either doing nothing or using the spin attack, or the sword touches and damages an enemy, dealing sword slash damage.
Spirit Sword can be used to open Spiked Chests and Mimics without taking damage.
Range upgrades move the hitbox of the sword forwards, including the spin attack. Enough range upgrades can leave a large blindspot directly in front of the sword.
The function of this item is a reference to the Master Sword in many The Legend of Zelda games.
When reaching 30.00 Tear Rate or higher, Spirit Sword will continuously use the spin attack, only stopped by holding down a fire key. This is likely due to the fact that starting at 30.00 Tear Rate, the internal number is "0", and begins to go negative beyond that.