Electric Vengeance

As the creature strikes you, an arc of lightning springs from your body and blasts the creature in return. Make a melee spell attack against the creature that damaged you. On a hit, the target takes 3d8 points of lightning damage. 

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the damage increases by 1d8 for each slot level above 1st.

"When ManDragor uses this spell his dragon mark absorbs the spell and the next time he casts it he can choose to deal the same damage as last time. (he can not store it a second time until he has taken a long rest.)