Blood Oath

Blood Oath is a passive item added in The Binding of Isaac: Repentance.

Effects

 * Spawns a familiar that stabs Isaac at the beginning of each floor, if Isaac has more than half of a red heart. This will drain as many red hearts as possible without killing him (leaving half a red heart if Isaac only had red hearts, or none at all if Isaac has soul/black/bone hearts).
 * Even though the animation is only a single stab when the effect triggers, it is treated as taking damage one time per half heart lost. This can lead to great synergies with items that activate on damage taken.
 * For the rest of the floor, Isaac gains +(0.15 * HeartsLost ^ 2) damage and +(0.1 * HeartsLost) speed, where HeartsLost is the total hearts of damage taken from Blood Oath (half a heart counts as 0.5).
 * This amounts to a max of +19.8 damage and +1.15 speed if starting the floor with 12 filled hearts (of which only 11.5 are drained).

Synergies

 * Activates the damage up effect of Adrenaline for the resulting empty heart containers at the start of each floor.
 * : Adds 2 full hearts' worth of bonuses if any health is drained at all, e.g. losing 1 heart gives the bonuses of 3 hearts.
 * Damage-triggered items: Activate a number of times equal to the total half hearts drained.
 * Does not activate the Panic Button trinket.
 * Each half-heart lost has a chance of spawning a rainbow poop, allowing for full red heart regeneration.
 * /: Each half-heart lost has a chance of spawning a full red/soul heart, allowing Isaac to heal back some of the damage immediately if he has high luck or a large amount of hearts lost.
 * Gives Isaac a soul heart if he had only red heart containers upon entering a floor.
 * Red hearts can be stored for use on the next floor.
 * With enough patience, health can be safely restored each floor by waiting in the starting room.
 * Activates the Whore of Babylon effect at the start of each floor.

Interactions

 * : The stab will trigger the teleport if tears are charging.
 * : Not recommended. Spawns 6 troll bombs for each half heart removed, making death highly likely without explosion immunity or Safety Scissors.
 * Perfection: The forced damage will not destroy the trinket, or prevent its acquisition.
 * Stem Cell: The healing will occur before any health is drained.

Trivia

 * This item is used in Bloody Mary (challenge #37).
 * The familiar will appear much bloodier when a large amount of hearts are drained.

Seeds
A088 LA10 (Curse Room adjacent to spawn)