Astor's Aegis

Whenever a creature misses you with an attack roll, it takes Necrotic damage equal to 1d4 plus the amount by which the attack roll missed your Armor Class. While your current Hit Points are 5 or fewer, it instead takes Necrotic damage equal to 2d4 plus the amount by which the attack roll missed your Armor Class.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the Hit Point threshold increases by 5 for each slot level above 1st.