Homebrew Cantrip

[Sparky McDibben]

Hey y'all

I need help coming up with a homebrew cantrip for my player. She's a half-elf bard (College of Swords) starting at 1st level. Key point: This is a one-on-one game going to level 20. That said, she loves dual-wield characters, and wanted to see if she could homebrew a bonus-action cantrip.

Here's what I'm thinking about allowing:

Thunder Lash
Evocation cantrip
Casting time: 1 bonus action
Range: 10 feet
Components: V, S
Duration: Instantaneous

You create a whip that lashes out to a creature or object within range. Make a melee spell attack against the target. On a hit, the target takes 1d4 thunder damage.
The spell's damage increases by 1d4 when you reach 5th level (2d4), 11th level (3d4), and 17th level (4d4).

Specific questions:

1) Is this overpowered?

2) How much time, cash, etc, should she need to do create the spell? I was thinking of making her character acquire something rare (thunder bottled during a hurricane? The heart of a dead elemental?) as part of the creation process.

3) At what level should she be able to create it (thinking 3rd-4th)?

Thanks!

[Desteplo]

Bonus action cantrips are non existent for a reason. For a caster it’s free and repeatable

Sword bards already get dual wielding fighting style. With the dual wielding feat she can hold a whip in her other hand

If she wanted to be more magical with her combat she can take the “sword coast adventures guide” cantrips (but that discounts dual wielding)

If you didn’t care about that and just wanted your friend to have fun? Does it matter if it’s balanced if she’s having fun? It’s a 1 player game so there isn’t any reason to hold her back (since she won’t outshine anyone else)

If you need anything to balance just give enemies a few extra HP

[Sparky McDibben]

So I didn't think the damage would be a big deal. If she took Dual Wielder she could outscale the damage with an off-hand rapier until level 11. Past that, it shouldn't matter much given the short range. What I really need some advice on is the creation process, both mechanically and narratively.

[stoutstien]

So I didn't think the damage would be a big deal. If she took Dual Wielder she could outscale the damage with an off-hand rapier until level 11. Past that, it shouldn't matter much given the short range. What I really need some advice on is the creation process, both mechanically and narratively.
Have her discover a piece of lore somewhere about creating a spell like this. Thousand and one plot hooks with this one.
The first thing that comes to my mind is a crazy hermit sorcerer who's willing to teach then this cantrip in exchange for tracking down some rare spell components