You attempt to completely dominate a creature.  A creature you touch must make an Intelligence saving throw.  They take 4d6 necrotic damage on a failure, and their Intelligence score is reduced by an amount equal to the damage taken.  On a success, they take half damage and their Intelligence score isn't reduced.  If this reduction causes their Intelligence modifier to fall to 3 or lower, they become entirely under your control for the duration.  You can command them telepathically on your turn, without using any action.  If their Intelligence score is reduced to 1, you can immediately expend a 6th level or higher spell slot.  If you do so, the creature is under your command until their Intelligence score becomes higher than 9.  This score reduction can only be reversed by greater restoration, or a similar healing spell of 5th level or higher.