When Isaac takes damage, a black ring briefly appears around him, dealing tear damage at a rapid rate to everything touching it.
When an enemy is killed with the black ring, there is a 15% chance for it to drop a Black Heart.
Black Hearts will drop as soon as the enemy is killed. (In other words, the heart will drop at the beginning of the death animation rather than at the end of the death animation.)
When Isaac kills an enemy, gives a chance for a black ring to appear around where it once was, dealing Isaac's tear damage at a rapid rate to everything touching it. The chance scales with Isaac's luck. The base chance is 25%, with an additional 2.5% chance per point of luck, capping out at 100% at 30 luck.
Each piece of Envy/Brownie Corn grants a 15% chance for a Black Heart.
Synergies
Contagion/Toxic Shock: Can allow for substantial chains of enemy kills and rapid room clearing.
Homing tears: The black ring becomes homing, curving around to hit enemies.
Hemoptysis: Enemies hit by the sneeze will gain a curse that increases the damage the black ring deals to 2x and +3 flat damage. Enemies who die to the black ring under the curse will damage and spread the curse to nearby enemies.
Ipecac: Massively increases damage and poisons enemies. Causes the ring to explode upon contact with walls or obstacles. The explosions can damage Isaac.
Proptosis: The ring is much larger and deals 1.5x damage.
Proptosis: The ring is much larger but deals normal damage.
An Athame is a ceremonial blade, generally with a black handle. It is the main ritual implement or magical tool among several used in the religion of Wicca, and is also used in various other neopagan witchcraft traditions. The athame stands as one of the four elemental tools in Wicca; traditionally standing for fire.
A black-handled knife called an athame appears in certain versions of the Key of Solomon, a grimoire originating in the Middle Ages.
When Isaac takes damage, a black ring briefly appears around him, dealing tear damage at a rapid rate to everything touching it.
Black Heart.
When Isaac kills an enemy, gives a chance for a black ring to appear around where it once was, dealing Isaac's tear damage at a rapid rate to everything touching it. The chance scales with Isaac's luck. The base chance is 25%, with an additional 2.5% chance per point of luck, capping out at 100% at 30 luck.
The Ludovico Technique and 
Brimstone.
Maw Of The Void.
Envy and 
Super Envy. It is also effective against 
Brownie due to all the 
Brownie Corns it spawns.
Contagion/
Toxic Shock: Can allow for substantial chains of enemy kills and rapid room clearing.
Hemoptysis: Enemies hit by the sneeze will gain a curse that increases the damage the black ring deals to 2x and +3 flat damage. Enemies who die to the black ring under the curse will damage and spread the curse to nearby enemies.
Ipecac: Massively increases damage and poisons enemies. 
Proptosis: The ring is much larger and deals 1.5x damage.
Wiggle Worm: The ring has wavy edges.
E. Coli/
Flush!: Enemies that are turned into poop can’t create black rings.
Holy Mantle: Getting hit when the Holy Mantle shield is active prevents Athame from triggering its effect.
A Lump of Coal: The ring damage is halved.
Eve Unlocks 
Hush Unlocks 
The Binding of Isaac: Rebirth
