What is the Blood?
A Blood Bond, also known as a Blood Oath, is a supernatural link of fidelity and dependency of one individual (the thrall) on a vampire (the regnant), created by the consumption of vitae.
Virtually anyone can be enthralled through it, making the blood bond one of the most powerful vampiric tools. The strength of this link varies based on several factors, but it is incredibly difficult to break and likely will have many lasting effects on the thrall.
Someone becomes involved in a blood bond by drinking the vitae of a Cainite, forming a complete blood bond. The thrall becomes subject to the regnant and loves them completely as much as they are capable of loving anyone. The regnant is the most important person in the thrall's life and takes priority over all else; sufficiently weak-willed individuals will commit any act the regnant asks of them barring anything that would cause their own or their regnant's death. It may still be possible for the thrall to act against the regnant, but never in a way that harms the regnant and only temporarily and after an extreme effort of will.
The dangers of the Blood
Having an insanely loyal servant is not without its drawbacks, however. As a thrall constantly seeks the vampire in question. More than one vampire has been blindsighted by the assumption that their thrall has no power over them. One can easily imagine the lengths a thrall could take, especially when given access to their sleeping domitor. If one could get away with it, why wouldn't the thrall simply capture and lock their domitor up where meddlesome outside forces can't disturb them? After all, the vampire needs nothing but blood to survive eternity, all it takes is a matter of sourcing it.