How can I improve this homebrew magic item so that it remains balanced across a wide range of levels?

Question

I am considering the following homebrew magic item for my campaign:

Ring, rare (requires attunement)

While attuned to this ring, when you cast a cantrip that targets a single creature (but does not have a range of Self), you may choose to expend a spell slot to have that cantrip target an additional number of creatures equal to the level of the spell slot. Any additional creatures targeted must be valid targets of the spell, and no creature may be targeted more than once.

If the cantrip requires an attack roll, make a separate attack and damage roll for each creature. If the cantrip requires a saving throw, each target makes a separate saving throw but takes damage based on a single damage roll.

I really like how this feels for low level parties. For example, a level 3 wizard using a 2nd-level spell slot to attack 3 creatures for 1d10 damage each with a Firebolt seems perfectly reasonable. I am NOT happy, however, with the idea of a level 17 wizard using a 9th-level spell slot to attack 10 creatures for 4d10 damage each.

How could this magic item be improved so that it is still interesting for low level characters without being so powerful at higher levels? Or am I overestimating its usefulness at high levels?

Answer 1

The ring will not be overpowered at high levels, and is probably a poor use of resources.

As you've stated, a 17th level Wizard could spend a 9th level spell slot and use Fire Bolt to target up to 10 creatures for 4d10 fire damage each using your magic ring. But that Wizard could also cast Meteor Swarm and deal 20d6 fire and 20d6 bludgeoning to each creature within four 40-foot radius cirlces (DEX save for half). For raw damage output, the ring doesn't even come close, with average damage per target of 22 vs 140 (assuming failed saves). That's not even factoring in that Meteor Swarm is most likely going to hit a lot more than 10 enemies.

Now, Meteor Swarm may be less applicable in a dungeon scenario if there is insufficient room to drop a 40-foot radius AOE. A simple replacement would be Fireball for a much more manageable 20-foot radius AOE. Dealing 8d6 fire damage (28 on average, DEX save for half), it deals more damage than the enhanced Fire Bolt, and only requires a 3rd level spell slot. And if the Wizard is really trigger happy with the AOE, the School of Evocation's Sculpt Spell class feature allows you to exclude friendly creatures from AOE damage effects.

Use of lower-level spell slots

The magic ring is more useful to a high-level caster if they use it to "upcast" cantrips with low-level spell slots. Using a first level spell slot to deal 4d10 fire damage to two targets is more DPS than a standard first level spell could typically achieve. Using Burning Hands as a comparison, we have 22 vs 10.5 damage on average to each creature (assuming failed saves). Burning Hands can hit more than two creatures, but would have to hit >4 creatures to "out damage" the "upcasted" Fire Bolt.

The issue arises in that this magic item is outclassed by other magic items of the same rarity in damage output. A Wand of Fireball is also a rare magic item and also requires attunement, but allows the Wizard to cast Fireball without expending a spell slot. It has a finite amount of charges, but the Wizard also has a finite amount of first and second level spell slots. A third level Fireball easily out damages the "upcasted" ninth level Fire Bolt. Given this, I'd be hard-pressed to pick the magic ring over the wand.

Answer 2

Nesbitto’s answer details why you might not want to be worried about someone burning a high-level spell slot on the ring as you cited, but if you still don’t want to see that happen (or don’t want to see your players get excited at using the item and subsequently let down by the effect being underwhelming), try this:

Cap it out at 5th-level spell slots.

A number of spellcasting features - Flexible Casting for Sorcerers, Pact Magic vs Arcanums for Warlocks, Arcane Recovery and several Tradition features for Wizards - already possess this limitation, so it seems a fairly standard spot to draw the line.