Crippling Shot

Once on each of your turns when you damage a creature with an attack made using your hex gun, you can force it to make a Strength saving throw against your spell save DC or fall prone and have its speed reduced to 0 until the start of your next turn.

 You can use this invocation a number of times equal to your Charisma modifier (minimum of once), and you regain all expended uses when you finish a long rest.