Bitter Aegis

An barrier of freezing energy manifests before you, protecting you from harm. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and the creature that damaged you is sundered by vapours of freezing energy. It makes a Dexterity saving throw, taking 3d8 Cold damage on a failed save or half as much damage on a successful one.

Using a Higher-Level Spell Slot. The damage increases by 1d8 for each spell slot level above 1.