Arc Lightning Blade

Make a melee weapon attack with your finesse weapon against one creature within 5 feet of you. On a hit, the target suffers the weapon attack’s normal effects, and you may cause an arc of arcane lightning to leap from the target to a different creature of your choice that you can see within 10 feet of the original target. The second creature takes 1d4 lightning damage.

Both parts of this spell’s damage increase by 1d8 when you reach certain levels. At 5th level, the melee attack deals the weapon's normal effects +1d8 lightning damage to the target on a hit; the subsequent arcane lightning deals 1d4 +1d8 lightning damage to the second target. Both damage rolls increase again by 1d8 at 11th level (2d8) and again at 17th level (3d8).