Calculated Retribution

You surround yourself with a dampening magical field and collect the energy of a foe's attack to use against them. When you take damage from a weapon attack, you can end the spell to halve the attack's damage against you, gaining a retribution charge that lasts until the end of your next turn. By expending the retribution charge when you hit with a melee attack, you deal an additional 2d10 force damage.

At Higher Levels. You may use your reaction to halve the damage of an attack against you up to a number of times equal to the level of the spell slot used, gaining a retribution charge each time that lasts until 1 round after the spell ends.

You must still make Constitution saving throws to maintain your concentration on this spell, but you do so with advantage, or if you already have advantage, you automatically succeed.