A regnant or domitor is a Kindred who imposed a Vinculum on another individual. That individual is known as a thrall.
Thralls experience a powerful, unnatural love for their regnant, and acting against their desires is extremely difficult (though not impossible). The regnant can use certain Disciplines on their thralls without the need for eye contact. A single vampire can be regnant to multiple thralls, though it takes regular consumption of the regnant's vitae to maintain the Vinculum after it is established. For human thralls, this amounts one taste a year; for Kindred thralls, the ties of regnancy can last for decades between tastes.
The Final Death of the regnant is one of the few ways to definitively end a Vinculum.