OK. So you feel that the ability to do this a couple of times a day would be suitable for a 7th-level full-caster.
What level do you think would be suitable for a class ability with the same effects as the spell would be for a class that is not a full caster to be able to do twice per day?
I said I would make it a 5th-level spell. A full caster would have to be 9th level to cast a 5th-level spell two times a day. Further as I have alluded to earlier, a full caster would not have things like heavy armor or martial weapons and it would be a specific spell, it would count against spells known and would not be an extra 9.5 damage to any spell he wanted to cast.
For example, this caster would have in his spellbook a custom spell called "Fireball with enhanced damage to one creature" that spell would be a 5th level spell, it would count against his spells known. He would also have to know a 3rd-level fireball if he wanted to be able to cast a normal fireball. If he wanted an enhanced lighting bolt he would need to know another 5th-level spell known to do it with lightning bolt. This balances the ability by taking away the versatility and tying it to the specific spell in the same manner as the spells you are comparing it against. Arbitrarily adding damage is fundamentally different than adding damage to a specific spell.
I'm using the example with fireball to demonstrate that the ability is not excessive, because that is the one you picked out.
Regarding the damage, d8+5 seemed reasonable at that level, particularly since you regard AC so highly. We can say that they drop the shield and deal 2d6+5 if you prefer?
What "other riders or effects" are you worried about?
An extra 9.5 damage on a fireball is a full 2 levels higher in casting and it is not something that can be combined with another spell.
Mostly I am worried about breaking action economy regularly - allowing two actions on one turn repeatedly.
OK. Even when applied to a 1st level spell like Chromatic Orb you regard the resulting damage as equivalent to a 6th level spell?
That is actually quite handy because Disintegrate is a also a single-target spell, so we can compare the effects directly.
Does that seem fair?
When applied to chromatic orb it outdamages a paladins smite cast with the same slot by a full 33% and when you consider all the variations for different spells it adds far more variety to the effects and delivery. That is OP by quite a bit.
Even magic missile which is a low-damage spell, still does more damage than a 1st level smite does, while doing the best damage type in the game and letting you split it unerringly among up to 3 creatures.
So yes, casting chromatic orb using a 1st-leve spell slot for this is VERY overpowered. If you upped the slot and made it be cast 2 levels higher (use a 3rd-level slot to cast it as a first level spell) it would not be so bad in terms of damage disparity. I still think it would be very powerful because of the sheer variety but would not be op. This would mean with a half-caster you could not do it at all until 9th-level and not be able to do it with a 3rd level spell until 17th-level.
Interesting.
Aren't martial weapons generally equivalent to about +1 damage, and Heavy armour only 1 or two points of AC? You feel that those differences balance out the difference between full spell progression and only half progression?
They more than balance out the differences, because they do not make you choose between dex and strength
To start with your math is incorrect. In melee Martial weapons are worth 2 points of damage for a dex-based character (dagger 2.5 vs Rapier 4.5).
In strength-based melee weapons it is 2.5 damage without a reach advantage (staff vs greatsword) or 1 point with a reach advantage (staff vs glaive/halberd). Note I did not consider a Lance in this discussion because of the limited conditions it can be used effectively. This is a baseline, it gets worse when you consider armor as well.
Heavy Armor is 8 points of AC vs an unarmored character, or 10 with a shield. Further it is always on and does not cost any spell slots. Yes, you can build a high-dexterity character to be close to or even better than that with limited use abilities and spells but you have to invest a ton to do it, especially in dexterity itself, which reduces the variety of weapons you can use for damage and the effectiveness of your spells.
To put a number on this when we are talking about both of these at the same time - investing in dexterity to boost AC means you can't boost strength and can't use a staff effectively. So now you are not comparing a staff to a greatsword any more. Instead you have to go finnesse and now you are comparing a dagger to a greatsword. That is a whopping 4.5 points of damage difference. Boost strength to use a better weapon and your AC is no wear near what it is with plate. The guy with heavy armor gets both of these (high AC and high strength weapons)
Personally I would have gone for Extra Attack, but I'm extremely interested in why you believe that it is Martial Weapons and Heavy armour that make the difference.
See above discussion
OK. If you want to stick to combat performance, that is fine. - This is a combat ability after all.
Do you believe that full caster classes should be more powerful in combat than half-casters and martials? By roughly how much?
In theory no. But the Bladesinger is the most powerful subclass in the game by a substantial margin and we don't need an even better GISH that would be more powerful yet.
In short, they should be equal and that is why you whould not be able to swing a sword and cast an action spell on the same turn.
Don't EKs get Extra Attack, Heavy Armour, Fighting style etc?
Yes and they can not both cast a leveled spell and make an attack as the same action, even at 20th level. Finally they are a third-caster, not a half caster.
Shatter is the closest 2nd level spell to Fireball that came to mind: both being ranged circular AoE spells.
Yes, far more powerful than any other class can do. As I said above classes should be about equal.
Look at what other classes can do in terms of mixing sword and spell and base your build on that if you want it to be equal.
Hence the reason for the comparison: At the point at which this subclass would be dealing AoE damage by swinging as part of casting Shatter, the Bladesinger would be dealing AoE with Fireball.
The bladesinger is not taking 2 actions on his turn to do it, further at the level she gets fireball she can only even attack once per turn.
The bladesinger doesn't have heavy armor, and can use bladesong only 3 times per day at that level to get AC equivalent to this and out of bladesong has to burn spells to get that AC out of bladesong.
The bladesinger has to pump dexterity to keep his AC high, making her spell saving throw lower
The bladesinger only has proficiency in daggers, darts, slings, quarterstaffs, light crossbows and one other 1-handed melee weapon. She does not even have simple weapon proficiency.
And the bladesinger is the most powerful class in the game. I think this illustrates my point!
OK. In terms of class performance in combat: why?
Because it breaks action economy.
Also as a point - you can effectively do this with a fighter Eldritch Kingth using Action Surge starting at level 3. He can do it once per short rest. He can do it twice at 17th level. He could do the attack-shatter combo you noted at 7th level once per short rest and twice total per day.
Similarly an 11th-level wizard can effectively do this with contingency once a day (twice a day with a day to prepare), burning a sixth level slot and a known spell to do it. Even at level 20 he can't do this more than once per LR.
Why do you think this character should be able to do it more than that? When it takes an EK until level 7 to do shatter/attack and Bladesinger until level 11 (albeit with more powerful options available) - how can you say it is balanced when you want to do this more often than either, and at an earlier level to boot?
