Spawns a familiar that stabs Isaac at the beginning of each floor if Isaac has more than a Half Red Heart. This drains as much red health as possible without killing him, leaving a half red heart if Isaac only had red health, or none at all if Isaac has soul/black/bone hearts.
Although the animation is only a single stab when the triggered, it's treated as taking damage one time per half heart lost. This can lead to synergies with items that activate upon taking damage.
For the rest of the floor, Isaac gains about +(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 +20 damage and +1.15 speed if starting the floor with 12 filled hearts (of which only 11.5 are drained).
This increases to +21.8 and +1.2 if starting the floor with 12 filled hearts, at least one being a Bone Heart.
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 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.
Bozo: Each half-heart lost has a chance of spawning a rainbow poop, allowing for full red heart regeneration.
Gimpy / Old Bandage /Shard of Glass: 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.
Magdalene + Birthright: With full Red Health, Magdalene can achieve up to +46.3 damage and +1.75 speed, and up to +49 and +1.8 with at least one heart being a Bone Heart.
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.
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 if Isaac does not leave the room immediately after being stabbed by blood oath, and making the starting room unsafe.
Gnawed Leaf: Becoming a statue at the beginning of the stage will not drain any red health.
Perfection: The forced damage will not destroy the trinket, or prevent its acquisition.
Stem Cell: The healing will occur before any health is drained.
Tooth and Nail: Item being active while the dagger falls will not drain any red health.