blast of immobility

you send a blast of white light at a creature within range where they have to make a dexterity saving throw. if successful they will be knocked back 10 feet. however, if they fail the save then they will knocked incapacitated for 1 hour but they can make a constitution saving throw to try and throw off the effect. if the target does become incapacitated then you can cast vampire touch as a bonus action without using up any spell slots.