Unstable Frost

You cast a ray of frost aimed as a target within 60ft of you. make a ranged spell attack, on hit the target takes 1d12 cold damage and must succeed a Strength saving throw or be grappled by tendrils of ice until the end of your next turn. If you cast the spell again at a target already grappled by this spell and hit, it is considered a critical hit and the ice restraining them shatters, freeing them. 

At Higher levels: When you cast this spell using s spell slot of 2nd level or higher, the cold damage increases by 1d12 for each spell level aboue 1st.