Is the homebrew weapon attack cantrip 'Caustic Cleave' balanced?

Question

I have created some new cantrips which include making a weapon attack as there are only two official ones to choose from. I've tried to follow these principles based on Booming Blade and Green-Flame Blade:

• Melee weapon attack only, 5 foot range
• No unavoidable extra damage to the primary target at level 1
• Damage no more than 2 targets
• Spellcasting ability modifier doesn't have to affect the spell at all
• Only available on the Warlock, Sorcerer and Wizard spell lists

But with such a small amount of official content to compare them against, it's hard to say if they're balanced.

Is this spell balanced with Booming Blade and Green-Flame Blade?

Caustic Cleave

Conjuration cantrip

Casting Time: 1 action
Range: 5 feet
Components: V, M (a weapon)
Duration: Instantaneous

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails.

On a hit, the target suffers the attack's normal effects, and acid sprays from your weapon arc to a different creature of your choice that you can see within 5 feet of you. Make an attack roll against the second creature with the same attack bonus. On a hit, it takes 1d6 + your spellcasting ability modifier acid damage.

At 5th level, the melee attack deals an extra 1d6 acid damage to the target, and the acid damage to the second creature increases to 2d6 + your spellcasting ability modifier. Both damage rolls increase by 1d6 at 11th level and 17th level.

Answer 1

Reasonably balanced

The similarities to green flame blade make this a good comparison spell to look at balance.

In your spell, you've traded the automatic fire damage that GFB provides for an additional attack roll for acid damage. Fire is commonly resisted and I think asking for an attack roll for an non-fire type is a reasonable trade off. The attack rolls presents a chance to miss, but also a chance to crit!

You've altered the damage a bit, too. GFB starts with d8 for the first target and 2nd target only gets the modifier. With yours, it's now a d6, but the second target starts off with the d6.

In all, you've traded your autohit from GFB for a less resisted damage type, an attack roll, and an additional damage die. That's a good trade in my book if you don't want to deal with fire resistance.

If you wanted a second look at damage, you could drop it to a d4 to keep the extra die, or keep the d6 and drop the extra die.