Blood Oath 
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).
- This increases to +21.6 and +1.2 if starting the floor with 12 filled hearts, at least one being a Bone Heart.
Notes
- Isaac can avoid damage from Blood Oath with invincibility, but he won't gain the damage and speed increase if he does.
- Eternal hearts give the health up before Blood Oath stabs Isaac, draining that extra heart too.
- It's also possible to use (XI - Strength) XI - Strength at the beginning of a floor before getting hit by Blood Oath to have one more heart counted in the bonus.
- If Isaac has only half a red heart, the effect will not trigger at the start of the floor, even if Isaac has soul/black hearts. Having no red hearts at all also prevents the effect from triggering.
- Works with (Keeper) Keeper / (Tainted Keeper) Tainted Keeper leaving his health at one health coin.
- The damage caused by Blood Oath does not lower the chance for a (Devil Room) Devil Room or (Angel Room) Angel Room to appear.
- While playing as (The Forgotten) The Forgotten, red health will not be drained if
The Soul was being controlled when entering the floor.
- If Blood Oath is removed (such as with (Tainted Isaac) Tainted Isaac), any existing damage and speed bonuses will not be removed until moving to the next floor as normal.
- Upon getting stabbed, any items that trigger upon getting hit will activate multiple times depending on the number of hearts drained.
- If Isaac has rotten hearts they will only count as half a red heart when blood oath activates.
Synergies
Adrenaline: Activates the damage up effect of Adrenaline for the resulting empty heart containers at the start of each floor.
BFFS!: 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. This allows for up to 14 hearts' worth of bonuses.
- 14 hearts' bonus gives +29.4 damage and +1.4 Speed.
- Damage-triggered items: Activate a number of times equal to the total half hearts drained.
- Does not activate the (Panic Button) Panic Button trinket.
Bozo: Each half-heart lost has a chance of spawning a rainbow poop, allowing for full red heart regeneration.
Gimpy / 
Old Bandage: 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.
Scapular: Gives Isaac a soul heart if he had only red heart containers upon entering a floor.
The Jar: Red hearts can be stored for use on the next floor.
Placenta: With enough patience, health can be safely restored each floor by waiting in the starting room.
Whore of Babylon: Activates the Whore of Babylon effect at the start of each floor.
Interactions
Cursed Eye: The stab will trigger the teleport if tears are charging.
Curse of the Tower: Not recommended. Spawns 6 troll bombs for each half heart removed, making death highly likely without explosion immunity or (Safety Scissors) Safety Scissors.
Gnawed Leaf: Becoming a statue at the beginning of the stage will not drain any red health.