Homebrew spell acting as one-way mirror

Question

Yesterday I was thinking that, in many cases, the line of sight is an important strategic element of ranged combat, so I was wondering if it could make sense for my wizard to create a new spell that supports attackers with ranged weapons.

Now I am asking: is this balanced?

One-way Foggy Wall

3rd-level evocation
Casting Time: 1 action
Range: 60 feet
Components: V, S, M (a handful of ash)
Duration: 1 minute - no concentration needed

You create an immaterial contiguous flat surface up to 1200 square feet, with any orientation that may be also floating in the air. Looking from one side of the wall it seems like an impenetrable fog is behind it, while from the other side it seems just a slightly smoked transparent film. The wall can be freely crossed both ways as it was not there, and lasts for the duration of the spell.
The purpose of the spell is to block the line of sight only from one side of the wall, it has no other practical effect.

Answer 1

This should be higher level than third

This is an interesting idea. The first comparator that comes to mind is tiny hut. Tiny hut is also 3rd level, and allows shooting out of, but not into. The hut does not allow casting spells out of it but in exchange also blocks opponents from crossing it. The biggest difference however is that Tiny hut takes a full minute to cast. As most fights are over in 3-4 rounds, that makes it unusable for combat. This spell in contrast, can be used in combat.

A second comparator could be fog cloud, which is only 1st level (and is a suprisingly good spell for first), but fog cloud of course is fully symmetric in effect, and the whole point of this spell is its assymetry, so I think that comparison is not as useful.

A third, maybe less obvious but very useful comparator would be greater Invisibility. It fundamentally has the same effect that the opponents cannot see you, but you can see them while you can attack and cast spells. Greater invisibility is considered a really strong spell for level 4, but it requires concentration (a big cost) and it only affects one target. This spell both does not require concentration, and can cover your entire team. In many situations, it's like mass greater invisibility that does not cost concentration.

While you can try to move through this one-way mirror to surmount it that way, that is not too hard to stop by placing some frontline fighters in the spaces directly behind it. These fightes also will benefit from being unseen while seeing their opponents, gaining advantage on their attacks and imposing disadvantage on attacks against them.

I think that the combination of being castable in combat without preparation and being able to cast spells that require line of sight through this one-way for the whole team makes this a lot stronger than tiny hut for combat, and even stronger than greater invisibility. Even in the worst case, it will give your entire team advantage on their attacks, give the entire other team disadvantage, and most likely it will severely nerf if not entirely blank opponent casters.

Third level is too low. I would make this at least fourth, probably better fifth, and maybe even sixth.

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Some technical details:

• there is no need to call out the absence of concentration in the duration, a simple "1 minute" is sufficient.
• You should clarify if the surface has to be in a single plane, or can be bent. Other spells that create such surfaces, like wall of force, describe as making them up of a number 10 x 10 foot panels -- that is simpler to track than overall square feet. You can look at such a spell for the exact formulation, reusing it makes it easier to understand and more consistent for people who know the other spells already, and avoids introducing unforseen issues.
• You also can probably drop the last line about the purpose, because who knows to what purpose crazy PC spellcasters will deploy this.

Answer 2

As written, this is far from balanced

One-way foggy wall is one-way.

This is a very powerful effect. Compare to magical darkness and having vision that sees through magical darkness, generally considered a very powerful effect. No one spell by itself provides this, especially for a whole party.

One-way foggy wall provides one-way heavy obscurement for the whole party.

A heavily obscured area--such as darkness, opaque fog, or dense foliage--blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.

In a sense, if used tactically, it blinds the entire opposing enemy.

One-way foggy wall is huge.

At 1200 square feet, you can create a one-way visual barrier that is 20 ft high and 60 ft wide. That is a huge space that gives you entirely a strategic advantage and no disadvantage. It doesn't block your vision. It doesn't block your ranged weapons. It doesn't block your spells. But for your enemy, it blocks them all.

One-way foggy wall does not require concentration.

Any even remotely similar spell also providing heavy cover (examples: fog cloud, darkness, sleet storm, and stinking cloud) requires concentration.

How to balance

Some combination of:

• Make it smaller - maybe 10 x 15, similar to darkness.
• Require concentration.
• Raise the level.
• Make it illusion and provide a save.
• Allow truesight to see through it.
• Require an expensive material component that is consumed.

Whatever you come up with, compare with existing spells, to the extent that you can.

For instance greater invisibility is 4th, and provides strategic advantage for only one creature, and requires concentration. Even with some of the restrictions above, in many situations this is much better than greater invisibility, which suggests that even if you restrict it, it's still at least 4th level.

Thought experiment

Play chess with yourself. Imagine two parties, of equal power, except one party has one-way foggy wall. Imagine the very best ways to use it. Imagine how you would defend against it.

If your goal is to support ranged attackers

Use greater invisibility. You can only use it for one creature, and it requires concentration, and truesight defeats it, and its 4th level, but it partially accomplishes your goal.

Improved greater invisibility

Greater invisibility can't be upcast, although invisibility can be. We don't know why it can't be upcast, but it is easy to see how powerful that would be.

Imagine an improved greater invisibility that can be upcast. Even at one additional creature per level, that is still pretty powerful. Maybe you can get your DM to sign off on this one. I'm not sure I would, though. Maybe if it were also 5th level. Even then, I'd want to playtest it.

Answer 3

The Spell Could be Considered Balanced

TL;DR: Not having concentration and the effects of "blocks line of sight" are what bother me.

To start, let's look at some comparable spells.

• Fog Cloud, which if cast as a third level spell obscures the friendlies in the area and requires concentration. Because of the third dimension, 60ft radius does give it amazing surface area by comparison. If your spell is 6ft tall (cover most character height), that still gives you a wall 200ft long, which is roughly a 32ft radius ring. So roughly half the dimensions if we're using a two dimensional map.
• It matches the area of Wall of Fire from a purely mathematical standpoint (60x20) which is a 4th level spell but also has the damage behind it, and requires concentration but has the one-way effect.

Now the illusion spells.

• Level 2 we have Phantasmal Force (PHB264) which affects one target, limits to a ten foot cube but gives you the base effect you desire. It requires Concentration.

• Level 3 Major Image which gives you a 20ft cube of "visual phenomenon" for up to 10 minutes but requires concentration.

• Level 4 we have Hallucinatory Terrain which gives you a 150ft cube of look/smell/sound like a terrain of your choice for a day, but has a 10min casting time.

Notably the "field" type illusion spells above break when interacted with and affect allies.

• Invisibility does a lot of what you want but only affects two creatures if cast at level 3 and breaks on the first attack. One hour but needs concentration.
• Great Invisibility is level 4, requires concentration, only affects one friendly for up to a minute and doesn't break on action. Note: With the invisibility spells, it has the benefit of moving with the target(s), but does not inherently let people see obfuscated allies.

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The main factor that puts me in favor of the spell being balanced is if the breakage of LOS is just about the disadvantage on attack rolls and not actual total cover. Invisibility rules still allow spell effects to target a space they can see which if they target the (flat) screen feels like it amounts to disadvantage to opponents if they have to make a spell attack roll but doesn't actually change anything if they're within an AOE.

Answer 4

Yes

Despite its inflexibility relative to fog cloud, your spell's area of effect and lack of concentration make 3rd level a good estimate for its quality.

Your spell is less flexible than fog cloud

Despite what another answer says, this spell is directly comparable to fog cloud, and your spell's use-case will generally come up less often than that of fog cloud.

As a prolonged aside to clear up an extremely common misconception, fog cloud is indeed asymmetrical; targets inside the cloud cannot be seen, but creatures can see out of the cloud without issue. This is because fog cloud creates a heavily obscured area. "A creature effectively suffers from the blinded condition when trying to see something in [a heavily obscured] area," but creatures in a heavily obscured area are not blinded. This is also how darkness, fog, and foliage (the examples of heavily obscured areas given in the PHB) work in real life.

In fewer words, fog cloud can produce the same effect as your spell, but in a sphere around the party, protecting them from all sides. This is far more flexible than the single plane your spell produces.

But your spell covers a large area

Despite its relative inflexibility, the sheer amount of size your spell can cover is quite powerful in very specific circumstances. For example, your spell could protect an entire battalion from enemy casters on level terrain, which is quite good. This is a niche use-case, so the spell is probably not fit for level 4 status, but certainly too strong for level 2.

And your spell does not require concentration

This is very attractive to a caster, and certainly makes the spell more valuable. It's doesn't make the spell better than a 3rd-level fog cloud, though, so I do believe that level 3 is a good estimate.

Spell balance is a matter of opinion

As a reminder, utility spells like this are often situational and often cannot be directly balanced against one another. Some low-level spells end up being overly strong in certain adventures, and some high-level spells are utterly useless in others. You're in the right ballpark, and that's as close as you can really get with a spell like this.