Thrall Instructor (5e Feat)
From D&D Wiki
Prerequisites: The ability to control any type of undead permanently, The ability to cast at least one spell, Intelligence 17 or higher
Once per seven days, you can teach any undead that is permanently in your service one spell that you already know. You cannot cast the spell that is taught to a servant again until one day has passed. You cannot teach the same thrall more than two unique spells and one cantrip. The undead can cast the spell once per 24 hours, provided it is not a cantrip. If the spell is a cantrip, the time limit on the feat is shortened to once per three days, and the servant can cast the cantrip at any time.