Mom's Knife

Mom's Knife is a Special unlockable passive item.

Effects

 * Isaac's tears are replaced by a knife that pierces enemies and can travel through obstacles. Holding down the fire button increases its range and power.
 * The knife can hurt enemies while Isaac is holding it.

Synergies

 * Brimstone - Firing the knife also shoots a barrage of knives that goes across the screen. The longer the charge, the more knives are added. The knives that are shot do 60% less damage than the standard 2x damage of the unthrown knife. (ex. base 3.5 damage does 7.0 unthrown, while the brimstone-knives do 2.8). This effect does not work with Azazel's default mini brimstone. The brimstone-knives do not benefit from A Lump of Coal.
 * Chocolate Milk - Damage is increased relative to the amount the knife is charged. Knife will also be colored brown.
 * Continuum: The knife can now travel through walls and appear out of the opposite wall but will still have the boomerang effect.
 * Epic Fetus - No effect.
 * Each explosion shoots 10 knives in a circle from the point of impact.
 * Eve's Mascara - Damage is heavily increased. However the knife takes longer to charge.
 * Fire Mind - Still applies burning but no longer explodes.
 * Ghost Pepper: The flames can shoot from Isaac while holding the fire button.
 * Gnawed Leaf - The held knife can damage enemies while Isaac is invincible.
 * Godhead - The knife homes in on enemies, but it does not gain a damaging halo.
 * Guppy transformation: Knife hits will rarely spawn an attack fly.
 * Homing Tears - Knives home in on enemies and colors the knife purple. It will stick on enemies for a second and make small lunges towards other enemies.
 * Ipecac - Knife turns green and may poison monsters that hit it.
 * Also adds +4 damage.
 * Loki's Horns - Four knives appear around Isaac and shoot out in each cardinal direction from Issac periodically. Just like Mom's Eye, this item is affected by luck.
 * Lost Contact - The knife will block bullets, even when held. Furthermore, the knife will not vanish after canceling a bullet, and therefore can cancel out several bullets at once while being shot or held.
 * Mom's Knife - If picked up twice in the same run, two knives are fired at once.
 * Mom's Eye - Two knives appear around Isaac and shoot in front and in the back of Isaac. This is affected by luck; the higher Isaac's luck is, the higher the chance the other knives will fire as well. The knives can also be used as melee weapons and will spin around Isaac as he turns around.
 * Monstro's Lung - Fires a cluster of knives.
 * Mutant Spider - Isaac shoots four knives. However, the attack seems to charge much slower.
 * Mutant Spider + The Inner Eye - Isaac shoots seven knives
 * My Reflection - No effect.
 * Proptosis - The Knife deals double damage regardless of range.
 * Strange Attractor - Enemies and pickups are attracted to the knife, even when not charging or firing.
 * Tammy's Head - Fires 10 knives in a circle around Isaac, dealing massive damage.
 * Tech.5: Both fire normally.
 * Tech X: Both fire normally.
 * Technology 2 - While charging Mom's Knife, Technology 2 will still fire, dealing significant damage.
 * Poison Tears - Knives gain a chance to poison enemies.
 * The Inner Eye - Isaac fires three knives instead of one that fan outwards. This also increases the charge time substantially.
 * The Ludovico Technique - Isaac controls a floating knife. Damage is greatly reduced.
 * The Ludovico Technique + Mom's Eye - Isaac controls a floating knife surrounded by 1 rotating knife.
 * The Ludovico Technique + The Inner Eye - Isaac controls a floating knife surrounded by 2 rotating knives.
 * The Ludovico Technique + Mutant Spider - Isaac controls a floating knife surrounded by 3 rotating knives.
 * The Ludovico Technique + Lost Contact - Floating knife loses almost all ability to block bullets. Only blocks shots that cross a very specific, small point on the knife. (Seems to be close to where hilt meets blade on the sprite.)
 * The Ludovico Technique + Loki's Horns - Loses Loki's Horns effect; Isaac controls only one floating knife.
 * The Wiz - Grants two knives that fire diagonally.
 * Trinity Shield - Blocks bullets in the direction the knife is held, giving enough protection to walk up to and melee enemies, or just to safely shoot the knife.
 * Unicorn Stump and other invincibility items - Even though the knife cannot be fired, the knife will deal damage while Isaac is invincible.

Interactions

 * Anti-Gravity: Overridden by Mom's Knife.
 * A Lump of Coal - Gives no damage boost but colors the knife dark brown.
 * Cricket's Body: Greatly reduces Mom's Knife range.
 * Piercing Tears: No effect.
 * Cursed Eye: Overridden by Mom's Knife.
 * Dead Onion: No effect.
 * Death's Touch: Increased damage and decreased tear rate. No scythe visual effect.
 * Dr. Fetus: Overridden by Mom's Knife.
 * Both the knife and the bombs fire normally.
 * Explosivo: Overridden by Mom's Knife.
 * Eye Of Belial: Overridden by Mom's Knife.
 * Holy Light: No effect.
 * Marked: No effect.
 * Monstro's Lung: Overridden by Mom's Knife.
 * Firing a knife causes several other knives to shoot out at random directions.
 * Mysterious Liquid: Overridden by Mom's Knife.
 * Number One: Greatly reduces Mom's Knife range.
 * Rubber Cement: Overridden by Mom's Knife.
 * Scorpio: Overridden by Mom's Knife.
 * Soy Milk: Damage drastically reduced at the benefit of a slightly better fire rate.
 * Technology: Overridden by Mom's Knife.
 * Both the knife and the laser fire normally.
 * The Parasite: Overridden by Mom's Knife.
 * Tiny Planet: Knives go in a spiral around Isaac; the longer the knife is charged, the bigger the spiral. Not recommended, as this makes Mom's Knife almost impossible to aim.
 * Tractor Beam: When Isaac changes direction, the knife snaps into the direction Isaac is facing.

Trivia

 * This item is used in Challenge #26 (I RULE!).
 * In the original Binding of Isaac, buying the item from the Devil Room cost 1 heart container instead of 2.

Seeds
8CKM J108 (Treasure room adjacent to spawn) 6PG9 7RK3 (Treasure room up and to the right one room from spawn) DVNV YV9Y (Treasure room adjacent to spawn) JW43 H8EK (Treasure room adjacent to spawn) 1M9E Y1G1 (Treasure room adjacent to spawn) P8SE A9JW (Treasure room adjacent to spawn) S63T 434W (Treasure room adjacent to spawn)