Balor's Mark

With this spell, you attempt to bind any sapient creature except a celestial, an elemental, a fey, or a fiend to your service. The creature must be within range for the entire casting of the spell. At the completion of the casting, the target must make a Charisma saving throw. On a failed save, it is bound to serve you for the duration. If the creature was summoned or created by another spell, that spell's duration is extended to match the duration of this spell.

A bound creature must follow your instructions to the best of its ability. You might command the creature to accompany you on an adventure, to guard a location, or to deliver a message. The creature is marked with Balor's sigil, a pair of curved horns, on your choice of the shoulder, wrist, forehead, or stomach. The mark is not visible unless the bound creature willfully reveals it, and may do so as a Free Action. The bound creature cannot be forcefully compelled to do so with Geas or any similar spell or ability.  The spell cannot be removed, except with the wish spell.

A bound creature cannot leave the material plane, except to travel to and from the feywild.  A bound creature attempting to enter a demi-plane, such as a space created by a Bag of Holding or Mordenkainen’s magnificent mansion must make a DC10 + Spellcasting Modifer Constitution saving throw or suffer 2d10 force damage. On a success, they may enter the demiplane normally, but must make another saving throw if they do not leave within 24 hours of entering.