Spawns a familiar that stabs Isaac at the beginning of each floor, draining all but half a heart of his red heart containers (or all of them if Isaac has at least one soul/bone heart).
Gives Isaac +(0.15 * HeartsLost ^ 2) damage and +(0.05 * HeartsLost) speed for the rest of the floor, where HeartsLost is the total half-hearts of damage taken from Blood Oath.
This amounts to a max of +19.8 damage and +0.6 speed.