Causes Isaac to fire dark burgundy-colored tears that have a 22% chance to doom enemies for 6 seconds.
Doomed enemies will have a number counter, starting at 3, above them. This counter will tick down when the enemy is damaged by Isaac and upon hitting 0, the enemy takes 5× Isaac's damage and the effect is removed.
The chance to inflict doom with a tear increases with luck, reaching a 100% chance at 34 luck.
Notes
The damage of the burst when the doom counter hits 0 is based on Isaac's initial damage stat when the effect was first applied, and if Isaac's damage stat changes since it was first applied, the damage of the burst will not be updated to reflect this.
Mom's Knife: The knife will be dark burgundy in color, and enemies hit by the knife can be doomed.
Dr. Fetus / Epic Fetus: Enemies hit by explosions from Dr. Fetus bombs and Epic Fetus missiles can be inflicted with doom. Dr. Fetus bombs will also be dark burgundy in color.
Brimstone and Technology 2 will both decrease the counter at a rapid rate, allowing the player to deal high damage very quickly.
Aquarius: Creep will be dark burgundy in color, and enemies that stand on the creep can be doomed.
Abyss: Absorbing Toy Piano spawns a black Abyss Locust that can doom enemies it hits. The chance to inflict doom follows the same luck-based formula as Toy Piano's tears.
Second Hand: Increases the duration of the doom status to 12 seconds, and increases by 6 seconds for every additional Trinket Multiplier.
Nyx: Cast Gems embedded inside enemies create a circular aura that dooms enemies inside its radius.
Video Showcase
Trivia
Toy Piano's flavor text is a reference to the tritone described in music theory, which has been historically referred to as diabolus in musica (Latin for 'the Devil in music').
Mom's Knife: The knife will be dark burgundy in color, and enemies hit by the knife can be doomed.
Dr. Fetus / 
Epic Fetus: Enemies hit by explosions from Dr. Fetus bombs and Epic Fetus missiles can be inflicted with doom. Dr. Fetus bombs will also be dark burgundy in color.
Brimstone / 
Technology / 
Technology 2 / 
Tech X: Lasers will be dark burgundy in color, and can doom enemies they hit.
Aquarius: Creep will be dark burgundy in color, and enemies that stand on the creep can be doomed.
Abyss: Absorbing Toy Piano spawns a black Abyss Locust that can doom enemies it hits. The chance to inflict doom follows the same luck-based formula as Toy Piano's tears.
Second Hand: Increases the duration of the doom status to 12 seconds, and increases by 6 seconds for every additional 
Trinket Multiplier.
Nyx: Cast Gems embedded inside enemies create a circular aura that dooms enemies inside its radius.
