Blue Lightning

You brandish the weapon (or weapons if dual wielding) used in the spell’s casting (can be used in tandem with a spiritual weapon). Blue lightning crackles and wreaths around the blade. When a melee attack is made against one creature within choses weapon range (5-10 feet generally), the attack deals the weapons regular damage and effects as well as an extra 3d8 lightning damage from the blue lightning. On a hit or miss you can cause bolts of lightning to burst out of the blade and strike up to 2 additional creatures within 10 feet, taking 1d8 lightning damage. 

*Optional* On a hit (full attack not bolt burst), target rolls constitution save against spell save DC, if fails, they are unable to move or react until the start of their turn

At Higher Levels. At 5th level, the melee attack deals an extra 2d8 damage and the lightning strike does an extra 1d8 damage to chosen targets (on a miss). One additional target may be chosen for the lightning strike (3 total additional now). At 11th level the damage to the main melee attack increases by 2d8 at 11th level and the lightning strike increases by 1d8 (7d8 and 3d8), also one additional creature may be targeted (4 now, on a miss of the main target) and 17th level follows suit for a maximum base damage of 9d8 for the main melee attack and 4d8 for the lightning strikes and a fifth and final creature may be targeted (5 creatures can now be targeted when the main attack misses). 

*Optiona* (even though this is against the rules for cantrips, in case one wants to make an all out attack or wants to make this into a 1st-3rd level spell as opposed to cantrip) Unique upcast ability: When upcasting to first level and beyond, the melee attack deals an extra 1d6 damage per level and the lighting strike attack also deals an extra 1d6 per level up to 9th level.