Downy Unstopables is not a laundry detergent itself so yes you'll need to use your regular laundry detergent along with the downy Unstopables.
Downy Unstopables are a scent booster and are meant to just make your laundry smell good for a longer period of time but it doesn't replace your laundry detergent.
So if you want clean clothes you'll need to use laundry detergent along with the Downy Unstopables.