Tonight I brought out my shadow blade and my GM ruled that I didn't get to add any extra damage to my shadow blade from my dexterity, because spells only do what they say they do, which is fine and I rolled with it.
My contention is that the spell says I should be able to add my dexterity to damage rolls since it is a finesse weapon. Was it the intent of this spell to deal 2d8 or 2d8+dex on a hit?
For those who want to know: my GM let me use my spell casting ability + PB for attack rolls.
Shadow Blade says:
You weave together threads of shadow to create a sword of solidified gloom in your hand. This magic sword lasts until the spell ends. It counts as a simple melee weapon with which you are proficient. It deals 2d8 psychic damage on a hit and has the finesse, light, and thrown properties (range 20/60).
Finesse says :
When making an attack with a finesse weapon, you use your choice of your Strength or Dexterity modifier for the attack and damage rolls. You must use the same modifier for both rolls.
Best Answer
Your GM is wrong
The general rule is that, when attacking with a melee weapon, the character can add their Strength modifier (or optionally Dexterity modifier, for Finesse weapons) to damage rolls. From Player's Handbook, page 196, "Damage Rolls":
From page 194, "Modifiers to the Roll":
The spell conjures, explicitly, a weapon, but makes no specific exception to the general rule, so the normal logic for attack and damage rolls is used. Since the weapon has the Finesse property, you can and must use either Strength or Dexterity modifier to make the attack and damage roll.
It's also worth noting that all the weapons in the Handbook are described by their damage dice only; for example the greatsword is listed as dealing "2d6" damage, not "2d6 + strength modifier".