Do I get to add my dexterity bonus to my damage rolls with Shadow Blade?

Question

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.

Answer 1

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":

When attacking with a weapon, you add your ability modifier -- the same modifier used for the attack roll -- to the damage.

From page 194, "Modifiers to the Roll":

The ability modifier used for a melee weapon attack is Strength, and the ability modifier used for a ranged weapon attack is Dexterity. Weapons that have the finesse or thrown property break this rule.

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".

Answer 2

I agree entirely with @kviiri's answer, but here's an alternate argument with the same conclusion. Look at other spells that create weapons and see how they're worded.

Do not use weapon rules:

• Blade Barrier, Cloud of Daggers, Guardian of Faith, Magic Missile: doesn't use the word "weapon", and specifies a non-attack method of damage.
• Conjure Barrage, Conjure Volley: uses the word "weapon", but specifies a non-attack method of damage. No way to make a weapon attack in the instant that they exist.
• Flame Blade, Melf's Acid Arrow, Mordenkainen's Sword: doesn't use the word "weapon", and specifies "spell attack".
• Spiritual Weapon: uses the word "weapon", but specifies "spell attack".

Use weapon rules:

• Creation, Fabricate, Swift Quiver: implies that you follow the usual rules for whatever kind of thing you create.

A special case for comparison:

• Shillelagh: explicitly says "use your spellcasting ability".

It's true that "spells only do what they say they do".

Shadow Blade does not use the words "spell attack" or "spellcasting ability". Instead it explicitly says that it counts as a proficient weapon with the finesse, light, & thrown properties. Therefore, to address your point on using spell casting ability and proficiency bonus, that is also an incorrect ruling based on the rules text. You must follow the rules for those weapon keywords, not the rules for spell attack keywords, and use your melee attack roll modifier (dex for finesse as appropriate) for this attack.

Answer 3

Jeremy Crawford, lead rules designer for 5e, answers this question. You can use Strength or Dexterity (your choice) for the attack/damage because it creates a finesse weapon for you. Thus all weapon and normal attack rules apply:

Shadow blade creates a melee weapon. When you attack with it, you make a melee weapon attack. Melee weapon attacks use your Strength modifier, unless a rule tells you otherwise. See "Attack Rolls" (PH, 194).

Keep in mind that because the shadow blade has the finesse property, you could use Dexterity instead of Strength. (6:42 Jan 3, 2018)