Crusius' Frost needle

You draw upon the elements to send a barrage of tiny needle-like pieces of ice at an opponent. Make a ranged spell attack against a creature when you can see within range. Upon a hit, the target takes 1d8 cold damage. Whenever the target moves willingly, the needles dig further into the body, causing more pain. When the target moves, it takes 2d8 cold damage. This effect lasts until the spell ends or the creature spends an action to dig the needles out.