Is the homebrew weapon attack cantrip 'Vampiric Assault' 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?

Vampiric Assault

Necromancy 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 if it takes any damage from the attack, you gain temporary hit points equal to your spellcasting ability modifier (minimum 1). The temporary hit points are lost at the end of your next turn.

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

Answer 1

This may be balanced

As this is only adding Temp HP and not actual healing, this is fairly well balanced. Instead of increasing damage, you are giving yourself a temp HP boost. I'd consider changing it to start of your next turn rather than end, though.

Otherwise, you basically get the free temp HP for the current round PLUS the next.

Answer 2

Strong, maaaybe unbalanced early, but fails to scale well

As you've noted, this is a pretty hard area to judge balance in. These cantrips occupy a unique place in the design space.

• They tend to do more damage/have more impact (if their special conditons are met) than comparable ranged cantrips like firebolt or ray of frost.
• This is balanced by the fact that their use is quite niche - if you're an arcane spellcaster you probably want to avoid being in melee since your defenses will probably be weaker than a martial character, and your primary melee attack stat is unlikely to be maxed making the initial melee attack less likely to hit. If you're a martial/martial-ish character who gets access to these (Eldritch Knight, Arcane Trickster, Bladesinger, Valor Bard), the fact that they don't allow you to use Extra Attack or Two-Weapon Fighting with them will make them less attractive options, not to mention if your spellcasting stat is not maxed/high-for-your-level the cantrips that use the spellcasting stat will be less good.

At levels 1-4

In this level range, it's comparable to Green-Flame Blade:

• Both have an extra effect keyed off spellcasting modifier
• No special requirement (like GFB's "another enemy within 5 feet) to activate it's extra effect
• The temp HP is useless if you don't take damage in the next turn, while extra fire damage to another enemy has a permanent effect on their HP

I think I would say that if you're using this cantrip in melee, you're going to get hit, and make use of the temp HP. How big a deal is this temp HP, then? Well, False Life is a 1st level spell that gives you on average 6.5 temp HP for an hour, no concentration. This cantrip gives you 3 temp HP (assuming point buy & min-maxed casting stat at level 1), at an opportunity cost of not dealing 3 extra damage to an enemy who may or may not be present. At level 1, 3 temp HP per round is a lot. Goblins, a CR 1/4 enemy who are often faced at level 1, deal an average of 5.5 damage on a hit. This temp HP cuts this damage in half, assuming you only take one hit per round. And a wizard/sorcerer/warlock will have 5-9 HP at level 1, making this temp HP between 50% and 33% of their total HP pool. That's a lot.

But maybe this is balanced by the fact that spellcasters really should not be in melee if they don't want to die at this level. Characters with stronger reasons to be in melee like Eldritch Knights and Arcane Tricksters only get this cantrip at level 3, by which time the temp HP is less of a big deal.

Levels 5+

Now the cantrip gets a lot weaker. GFB & Booming Blade get +1d8 to both their initial damage and their extra effects, while Vampiric Assault only gets +1d6 to the initial damage. Now granted, you will always be able to use Vampiric Assault's extra effect (unless you have temp HP from another source), but as levels scale up 3-5 temp HP will become less and less relevant.

My suggestion

The cantrip seems ok at levels 1-4. Possibly a little stronger than the other similar cantrips, but not so much I think it definitely needs changing. I think if you're using this in a game you should warn your players you may need to tweak this (perhaps with an agreement to let them switch cantrips if they're unhappy with the tweak), and watch how it does at these levels.

Beyond these levels, it needs a boost in its scaling.

I would suggest following the example of Green-Flame Blade and having matching scaling to both the initial damage and the extra effect (in this case, temp HP). My instinct is to go for d4s for this scaling, but that's just based on a gut feel of the impact of lots of temp HP every round at higher levels. I may be overestimating the power of that temp HP, in which case d6s would be appropriate.

Answer 3

What stops someone from using this on every attack before they get multiple attacks? It seems like it will just be free temp HP for no cost. If you compare it to Booming Blade and Green-Flame Blade, their effects can often not matter, but this is always helpful. I would suggest it can only "drain" temporary HP from a creature once.

Answer 4

I do not think this is balanced. If you compare it to the Samurai fighter's Fighting Spirit feature or to the Glamour bard's Mantle of Inspiration feature, those are basically on par for temp HP received, but they have limited uses.

This is basically a cantrip that gives you extra damage and lets you attack and pseudo-heals you - all at the same time - that you can use an unlimited number of times per day. Also compare to the vampiric touch spell, which only heals you an amount of HP equal to half the damage done.