When used, Isaac gains +1.0 speed and can walk through enemies/bullets for 1 second or until he shoots. If Isaac walks through an enemy or bullet during this time, it freezes until the effect ends, then takes damage equal to 2x Isaac's damage plus a flat 3.5. If there are multiple enemies/projectiles caught in the attack, each consecutive attack deals an additional flat 3.5 damage. At the end of the effect, an area of effect blast is created, dealing more damage the more enemies or bullets were pierced during the effect.
For every enemy/bullet Isaac walked through, he will gain a temporary +1 damage bonus, which wears off at a rate of -0.25 damage every half second.
The damage of the blast at the end of Dark Arts is equal to the sum of every damage dealt while dashing between enemies.
Bullets walked through will still count for the final blast damage calculation, even though they can't really be damaged.
The player can cancel the invincibility (and hence receive the damage bonus early) by firing tears
If an enemy/bullet Isaac walks through dies or otherwise disappears before Dark Arts deactivates, they will not give a damage bonus.
Explosive projectiles will explode when destroyed by Dark Arts, unlike those destroyed by Lost Contact.
Dark Arts will trigger certain tear effects on enemies it damages, such as burning from Fire Mind.
Dark Arts is extremely effective against enemies with multiple segments such as Larry Jr. because it can affect each segment individually, greatly increasing both the immediate damage and the temporary tear damage bonus.
Invincible enemies such as Stoneys and Stone Grimaces can still be affected and used to increase the damage bonus gained.
Hosts can be damaged and killed even while they are lowered and invincible.
Similarly, using Dark Arts on Scolex's invulnerable segments will still deal damage, making the fight significantly easier.
Orbitals that deal contact damage will still deal damage while using Dark Arts
Dark Arts can be a useful tool for Tainted Jacob to avoid colliding with Dark Esau, as the invincibility and stunning effects of the item fully work against him even if Esau charges at the player.
Dark Arts still works on reflected bullets.
Dark Arts works on friendly or charmed enemies. This means if an enemy that summons other enemies such as an Oob is charmed or captured by an item like Friendly Ball, it is possible to generate infinite damage.
120 Volt: 120 Volt will still activate while using Dark Arts, allowing Isaac to deal more damage to nearby enemies while invulnerable.
Aquarius: Due to the nature of Dark Arts, Isaac can easily damage enemies with the trail left behind.
Camo Undies: If used at the start of a room, after Dark Arts' effect ends, Isaac re-enters Stealth.
Car Battery: Dark Arts will last twice as long, totaling at 2 seconds per use. If you end Dark Arts early, a second Dark Arts will activate for the time that you had remaining.
Euthanasia: When Dark Arts ends, each enemy tagged may instantly die and release a burst of black tears as if hit by a Euthanasia tear.
Friend Finder: Dark Arts can be used on friendly mobs created by Friend Finder without killing them, but still giving you the damage boost.
Friendly Ball: Enemies which spawn other enemies can be captured and used to generate infinite damage, as the enemies they spawn will also be friendly.
Gnawed Leaf: If Isaac is stationary when Dark Arts ends, he will gain invulnerability without any downtime. Dark Arts can also be used from invulnerability without taking damage.
How to Jump: Jumping over an enemy or projectile while Dark Arts is active will tag it.
Ipecac: Because Dark Arts' damage is scaled off of tear damage, Ipecac gives an extremely high damage boost to each Dark Arts hit, easily capable of destroying some early bosses in a single hit.
Kamikaze!: Kamikaze! can be activated without harm when using Dark Arts.
Mars: Isaac is able to dash while using Dark Arts, allowing him to reach more enemies in one use of Dark Arts while still dealing damage with Mars' dash.
Mom's Knife: Mom's Knife will still deal damage even while in Dark Arts, killing most weaker enemies and bosses easily.
Montezuma's Revenge: The corns created by Montezuma's Revenge can be caught by Dark Arts, and most will naturally, making each cast of Dark Arts give huge boosts of tear damage, and making the final blast deal huge amounts of damage. Also, the Brimstone beam created will still be active while Dark Arts is active.
Pause: Activating Dark Arts will not cancel the effect, turning Pause into an instant room clear in most situations and an effective tool for damaging bosses, allowing you to get up to 4 uses in before they resume.
Pause + 4.5 Volt: Pause becomes an even more effective tool against bosses, as it can now be activated several times in the fight.
Pause + 9 Volt: Pause can be used every room, making most of them very easy to clear.
Rock Bottom: Rock Bottom will prevent your damage from decreasing, allowing you to keep the highest achieved damage bonus.
Taurus: Will be activated during the usage of Dark Arts, activating the invincibility in the same instant that Dark Arts is activated.
Wisps: Wisps cannot be destroyed while Dark Arts is in effect and still deal contact damage.
Book of Virtues: Spawns black wisps on the middle ring whenever an enemy is killed with Dark Arts. The wisps do not persist between rooms.
Familiars such as Cube of Meat that block projectiles will make it difficult to gain damage off of projectiles, as they destroy any projectiles they come in contact with.
Clots: Clots will gain increased speed and be no longer be able to fire, but will still be vulnerable to damage while Isaac is using Dark Arts. They will most likely hit any enemies or projectiles Isaac runs through if the player does not use the drop button to make them stay still.
Libra: Libra is extremely detrimental to how Dark Arts works, and should be avoided: The speed boost during the invincibility phase is transferred into attack bonuses (Which are useless as tears cannot be used), and afterwards, the temporary damage boost is divided across all of Isaac's stats, resulting in very negligible stat boosts.
Using Pause while there are enemies and also tagging the White Fire Place will extinguish it.
The item is a reference to a flash game Edmund Mcmillen created in 2005 called Grey-Matter. In this game, you would kill enemies in a fashion identical to Dark Arts.