Enforce Enslavement

While holding the mithril in one hand, put the drop of blood on your pointer finger and touch the subject between their eyebrows. The target must succeed on a Wisdom saving throw or be enslaved to the caster until dispelled. The spell has no effect if the target is undead, if it doesn't understand your language, or if your command is directly harmful to it.

On succession the mithril is removed from your hand and shaped into a collar which the slave will wear on their neck.

The spell can be dispelled by many means although there are only 2 widely known ways:

Death of the subject

A Skilled cleric and/or paladin casts bless using a 9th level spell slot and holy water. The subject must then make another wisdom saving throw to see if the spell succeeds.