How exploitable/balanced is this homebrew spell: Spell Ward?

Question

I am designing a trap for my players. One of them has Disintegrate (known to their enemies), which would make escaping the trap relatively easy. I would like to block this method, once. To achieve this, I have designed the spell below, and in the spirit of not overusing magic that's not available to the players against them, plan to make it available to them too.

Before implementing this spell in game I would like to be aware of the potential ways in which this spell could be unbalancing and/or exploitable. I have already considered making the spell a general (single-use) ward against any spell, and dismissed that as frightfully overpowered. I am aware that putting this spell in the world will increase the power available to my players (it's something they can set up prior to an encounter with no tradeoff in capabilities during that encounter) - my aim is that it not cause a very large increase in power because I missed an edge case.

Spell Ward

3rd level abjuration

Casting Time: 1 Action

Range: 120 feet

Components: V, S

Duration: Until dispelled

Choose any creature or object within range, and name a spell known to you. You place a ward on that creature or object. An unwilling target must make a Wisdom saving throw to resist the ward being placed on them.

While the ward remains on the creature or object, the next time they are targeted with the named spell or an effect of the named spell, they are unaffected and the ward is dispelled.

The ward offers no protection against spells that would be immune to Dispel Magic.

If Dispel Magic is named as the warded spell, no other spells on the warded creature or object are dispelled when the ward is dispelled.

So: how balanced/exploitable is this homebrew spell?

Here is a non-exhaustive list of possible changes I've thought about that could balance the spell if certain aspects are unbalanced.

• If the spell is unbalanced as an offensive/in-combat/shortly-before-combat tool:
  • Remove the ability to cast this spell on unwilling targets
  • Increase the casting time to 10 minutes
  • Reduce the range to Touch
  • Remove the special Dispel Magic case, stopping it from being a counter to an enemy dispelling effects
• If the spell is unbalanced as a counter to high level spells
  • Require an ability check like Dispel Magic's when the ward is hit by a spell of a higher level than it
  • Only allow the ward to affect spells of the same or lower level than the level it was cast at
• If the spell is unbalanced due to its infinite duration
  • Reduce the duration to 10 days, in line with Contingency
  • Require a material cost for the spell along the lines of Glyph of Warding

Answer 1

It's close, with some changes to prevent abuse.

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Duration

Given your intended use for this spell, the following suggestions have been made with the assumption that the effect lasts until activated or dispelled.

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Non-targeted spells

There are serious questions about spells that do not directly target a creature (fireball, wall of fire, and what about illusions like hallucinatory terrain?)
To prevent some of the more problematic cases, I recommend changing the possible spells to:

"Name a spell known to you that targets one creature or object."

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Stacking and Recasting

In theory, you could, given a lot of time, stack this spell (layers) or recast it.

Additionally, I can see this spell being ridiculous, in that a party with access to it will spend a day before any adventure prepping against the first cast of many different common spells. This can really get out of hand because this spell allows a third-level spell slot to neutralize a high-level spell as written.

To prevent party reliance and overuse of this spell, I recommend changing the duration to:

This effect lasts until activated or the spell is recast. When this spell is cast, any previously existing ward is immediately dispelled.

You may also add:

When cast at higher levels, the spell can target one additional creature or object per level. The warded spell must be the same for all affected creatures.¹

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Countering Higher-Level Spells

Given the infinite duration of the effect, I recommend limiting the effect:

The ward may only protect against a spell of the same or lower level as the spell slot used.

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Range

Most spells of this type don't have such extended range. I recommend reducing the range to 10 feet or touch.

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Casting time

The Ward effect starts to encroach on counterspell. I recommend keeping the infinite duration (above) but changing the casting time to 1 minute. This would allow the players to alter the warded spell in a tense situation, but not during combat.

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¹ That being said, it may be more balanced to make it "For every two additional levels, the spell may target one additional creature or object." I think this part of the spell would need to be playtested.

Answer 2

Have you considered Counterspell in a Glyph of Warding?

The Glyph can be triggered by the first casting of Disintegrate and would need to be a spell check against DC16. A 5th level caster would have about +4 on this, an 11th (which is at least where your players are to have Disintegrate would have about +5.

Alternatively, it could be by the first casting of any spell because players often surprise you with their choices. Also, it could be one of many designated to trigger on the first, second, third etc. so the players can engage in the fun pastime of “find the Glyph”.

This has the advantages of not requiring homebrew and not being an obvious “screw you” designed to nerf the players’ choices.

Answer 3

This is definitely a good idea for a spell, although maybe a bit overpowered. I always like the idea of a villain with one powerful spell that they can cast once to turn the tide of battle. If the players can negate this (probably around 9th level) spell with a 3rd level, that would probably make you regret some choices.

For that reason I wouldn't make it cover all the levels of spells, or even give them a chance with an ability check. You could instead choose the simpler way, having to use the same or higher level spell slot as the spell you are trying to block. This way the players are giving up a spell slot for one of the enemies spell slots. To make this work however, you would have to make it last until their next long rest so they can't just get use it in the evening and get their slot back.

To make the spell feel more like an investment you can add an item cost and a longer casting time so they can't just pop it mid combat.

To make the interaction between the ward and dispel magic more interesting you could have the spell slot you use for the ward as a baseline for anyone who tries to dispel something on you. For example: If you have the 4th level spell stoneskin active but you also have a 6th-level ward on you, someone who tries to dispel the stoneskin has to use a 6th-level (or make the ability check with DC 16) because of your ward. Stoneskin and your ward are then dispelled.

This is the way I would personally run the spell. Feel free to mix and match our versions as much as you like.