Is the homebrew weapon attack cantrip 'Astral Smash' 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?

Astral Smash

Evocation cantrip

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

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 it becomes outlined in sparkling light until the end of your next turn. While outlined, it sheds dim light in a 5-foot radius and cannot benefit from being invisible.

At 5th level, the attack deals an extra 1d6 radiant damage to the target. The damage roll increases by 1d6 at 11th level and 17th level.

Answer 1

Not balanced

Like your other one involving an effect lasting beyond, I think this would do better with ending at the start of your turn.

The Monk's stunning strike lasts until the end of their next turn, but there is a resource cost (ki) to do that. This has no resource cost.

Compare this to a paladin's branding smite. This is a concentration spell with a spell level resource spent.

Answer 2

So, kind of like Faerie Fire (a 1st-level spell) but:

• a cantrip
• a hit roll instead of a save (which allows for advantage to be used much easier)
• one round duration
• does not grant advantage
• does additional damage
• scales with even more damage

Or, looked at another way, it's GFB or BB, with a better damage type (though with the requisite reduction in die size) and trades conditional damage for a guaranteed invisibility purge.

That's a tough call. Invisibility purge is rather niche, but also quite clutch. I'd probably allow it since it only affects one creature at a time.

But as niche as it is, it'd probably be better to use a resource like a spell slot and not something potentially more limited like cantrips known. Honestly, I'd just make it a level 1 spell, something like this:

Illuminating Blade

1st-level evocation

Casting Time: 1 bonus action
Range: Self
Components: V, M (a weapon)
Duration: 1 minute

You impart a ghostly illumination to a weapon you wield. When you successfully attack a creature with the weapon, it is similarly illuminated.

While illuminated in this way, objects and creatures shed a dim light in a 10-foot radius and can't benefit from being invisible. Additionally, any attack roll against an affected creature has advantage if the attacker can see it.

This brings it more inline with Faerie Fire while still allowing the use of Strength or Dexterity-based attack rolls to deliver it. The duration is still capped at one minute, and you can only impart the effect to one creature at a time (more with Extra Attack and bonus-action attacks), but the compensating advantage is that you can repeatedly attempt to apply it if you fail the first time and you can eventually have a group of affected creatures, similar to FF.