Bahgtru's Blessing

Your body becomes imbued with Bahgtru's strength and destructive power. For the duration of the spell, you can use your spellcasting ability instead of Strength for your unarmed or  improvised weapon attacks and damage rolls. If you attack with a single fist or one-handed improvised weapon the damage die is a a d8. If both of your hands are empty, or you wield a two-handed improvised weapon, you can make a two-handed attack, which raises the damage die to a d10. This fist deals bludgeoning damage or lightning damage. you choose which type when you cast the spell. You can only make this attack once per turn or if an enemy provokes an attack of opportunity. All off-hand strikes deal normal damage. 

Note: This a flavor spell based upon the Shillelagh cantrip. It caps damage at level 5 for balance. Ultimately it is weaker than Booming Blade or Green-flame blade even at level 5, it deals 1d8 less damage.

Cantrip Upgrade. The damage increases by 1d8 when you reach levels 5 (2d8). Damage caps at 2d8.