So! This is my first DnD game. My best friend told me we should play it, cause I'm always coming up with stories for video games I'm planning. We've played about three sessions so far, had a lot of fun, even if it's a slow start and everyone is still learning how to play. We'll make it!

Anyway. One of my players is a dragonborn bard, and he really wanted to be able to play the piano, cause he plays it in real life and like the instrument. (I do too, I don't blame him) So after a while of thinking up ways that he could somehow have a piano with him at all times, I had the idea to replace one of his bard spells with a special made one.

But I have no idea how to go about balancing this. I tried to think of all the things he might try to do with it. And tried to make it slowly get stronger as his character learns how to use it better. I'm just not sure how well balance it is. I would really like it to be a nice utility spell. But I'm not sure what all I should be taking into account.

So far, he can use it as a spellcasting focus, use it as a springboard, use it as a shield, use it as a hail of projectiles, use it as a nova blast, and also use it to possibly make his enemies flinch even when the keys aren't solid, cause they would think they're about to be hit. (Have not written any rules for that part. Would probably be a wisdom save though.)

So anyways. Here it is. It's pretty long.

Thanks in advance for any feedback.

(Edit: general consensus was that the spell was too long and complicated, I have attempted to shorten and simplify it)

Casting time: 1 action

Range: 15 feet (stable)

Duration: half an hour (can be extended by casting again. The extension action does not interrupt playing the instrument.)

The caster takes an action and waves their hand to create ethereal wispy piano keys, numbered in octaves, and starting at 2. (12×octaves+1)

These keys make the sound of a piano when touched, and will adjust their volume depending on how hard they are pressed. The volume can far exceed regular piano volume levels, but cannot raise far enough to deal damage or affects. You may play them like a regular musical instrument, and use them as a bardic focus.

The keys can exist for 1.5 seconds outside of their 15 ft stable spell range. You may use a bonus action to fire any number of octaves at a single target 60/150 ft before they vanish, playing the notes, and dealing 1d4 force damage on a hit.

When they are pushed on with a large amount of force, you must make a concentration saving throw, or the spell is broken.

At higher levels: When you cast this spell using a higher level spell slot, you may summon one additional octave for each slot above 1st level.

That's not a spell, that's a subclass. 

Make it a lot simpler if you want it to be a spell. If you’re going for a damaging spell, make it a cantrip akin to Vicious Mockery, and flavor text in the fact that the Bard is playing an ethereal piano during the attack. Not game breaking, and pretty cool imo. Think like, that scene from Kung Fu Hustle with the people shooting blades out of their instruments

 This is definitely a bit complicated.  The easiest solution is to just give him a keyboard for a bardic instrument in place of the standard harp / mandolin, then refluff existing spells.  Wants to use it as a shield?  Okay, sure, one improvised shield - I think there's rules for that kind of thing.   Want to use it to nova blast?  Cast using Thunderwave or similar spell. 

At the furthest extreme, just make it as an Instrument of the Bards .

You're trying to make an entire character into a single spell/magic item, when it really should be a combination of everything the PC can do, plus their magic item.  Break it up a bit.

 So.. Three votes to make it simpler. Done!

I took out all the complicated stuff and made it more of a normal spell. I'm sure he'll try to use it as partial cover at some point, but I'm not sure if they should be able to do that.. I really liked the reaction shield idea too.. 

Anyway. I trimmed it up. It starts at 2d4 damage from a 1st level slot, and gains 1d4 for every extra level. I don't want it to be a very damaging spell, since he can spread that damage out over several rounds using his bonus action to attack. But I would like it to be a little more than just a spellcasting focus and some extra damage.

Oh, I think I forgot to add the cantrip rules. I wanted him to be able to summon a non-damaging version at-will, so that he could use it as a focus. Or.. Would it be acceptable to just allow the cantrip version to only be one octave? (1d4 damage)

I think even having the specific octave damage functionality makes it a pretty complex and hard to balance at any level. How about a simple, mostly non combat cantrip available only to a bard. This allows the flavour the player wants without risking a power imbalance. The only benefit to this cantrip is that the player can never lose their spell focus. 

Spectral Instrument (Verbal component only, duration 8 hours)

As a bonus action you summon an ethereal instrument, barely visible, which hovers beside you in your square and moves with you for the duration. The instrument may be of any type with which you are proficient, and it produces sound like a high quality example of that instrument. Only you may touch the instrument, and it becomes more opaque when you are playing it. A bard may use this instrument as a focus, but must have one hand free to touch the instrument. The instrument can also be used for a musical performance. You may dismiss the instrument at any time. 

So with that cantrip the player can imagine these silvery keys floating beside them at all times, ready to summon an illusion with a quick arpeggio, but it avoid messing too much with mechanics.