Is this homebrew "Mask of Illusion" spell balanced?

Question

The goal of the following spell is to cause observers to think that the target is an illusion. For example, you could cast this spell on yourself while entering a sleeping dragon's lair and if the dragon woke up and saw you, it would think you were an illusion, and perhaps decide to search for the caster of the illusion instead of bothering you for a while. Observers who are absent-minded or those who are ignorant of illusions are naturally less susceptible than the perceptive ones.

I would like to learn if it is balanced as a 5th level spell.

As a bonus: I would be happy to hear about possible ways to powerplay with this, and reasonable methods to curb that powerplay to keep it balanced without increasing its level.

_{Apologies for the long text and the unusual mechanics, some of the (admittedly naive) reasons are listed under the text of the spell as extra material at the end.}

Mask of Illusion

5th-level illusion [wizard]

Casting Time: 1 action
Range: 120 feet
Components: V, S, M (a bit of fleece, and a transparent gem worth at least 500 gp, which the spell consumes)
Duration: Concentration, up to 10 minutes

You cause an object, a creature, or some other visible phenomenon that is no larger than a 20-foot cube to appear like an illusion.

You create some subtle effects, which can include images, sounds, smells, and temperature modifications, on or around the target, that falsely hint to perceiving creatures that the target has been created by a spell like major image. For example, the voices of the target might sound like they are coming from a few feet away, the target's shadow might appear to fall in a slightly off angle, or its image might fluctuate in and out for a barely perceptible duration, etc.

For each creature that encounters the effects you generate, the GM makes a passive Wisdom (perception) check against your spell save DC, but you can opt to decrease the DC down to 11 by purposefully leaving more fake clues, in order to lure creatures more easily. Creatures who succeed in their checks will feel suspicious about the reality of what they see.

A suspicious creature that uses an action or a bonus action to examine the effects you create, can falsely decide that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC, which can again be lowered down to 11 if you desire. Success in the check means the creature falsely discerns the target as an illusion, as if they had succeeded in discerning the presence of a major image. They become able to see through the target, and the target’s other sensory qualities become faint to the creature.

Physical interaction with the target reveals it to be real, because unlike an illusion things cannot pass through the target. A creature that has come to accept the target as an illusion previously, but uses another action to re-examine the target can determine that they have actually been fooled with a successful Intelligence (Investigation) check against the spell save DC used in the earlier Intelligence (Investigation) check.

The spell is cast the same way major image is cast, so Intelligence (Arcana) checks to identify it are made with disadvantage. The gem that is consumed by the spell needs only be on the caster, and does not need to be touched or shown.

At Higher Levels: When you cast this spell using a spell slot of 8th level or higher, the spell lasts until dispelled, without requiring your Concentration.

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The rest of this post is meant to address issues raised in the comments and summarize some of suggestions.

They are listed here for future reference in case the comments get deleted.

• Renamed spell as "Mask of Illusion", since the original name "Virtual Illusion" was not giving enough information to the reader. (Thanks to @Molot.)
• The unusual backward mechanics of the first checks "punishing success" (as pointed out by @Miniman) is to allow the spell to simulate the gameplay of how 5e illusions are handled. A creative DM can use the spell to fool players.
• Answering a question by @Szega: Special senses like true sight or witch's sight just do what they say, they show the truth, so creatures with such abilities are immune to the effects of the spell.
• Answering a question by @enkryptor: I prefer to balance the spell at 5th level because it was initially designed for and playtested in AD&D 2e. In that edition beings with intelligence scores higher than 18 could immediately recognise illusions of levels up to Int−18. That rule could be used by the DM as a starting point to define what sort of creatures would be more susceptible for being fooled (but it was not limited to Int 18+ creatures in any way). 2e had Spectral Force at 3rd level, similar to 5e's Major Image. Spectral Force needed concentration and its no-concentration version was a 5th-level spell known as Advanced Illusion. This spell was unusual, but required concentration, so we went for Advanced Illusion's level. As some sort of insurance, we put the 500gp material component to limit its daily use (as we weren't sure if further gaming would give us some powergaming ideas). Yet, none of the powergamer casters desired to learn it. The spell was used sparingly & never caused any issues over years of gaming.
• The spell is probably most meaningful in the hands of a DM who likes the narrativist play style, where beings are intrigued by illusions and don't simply attack them on sight. A simulationist DM might use the translucency created by the spell to be limited to what the caster knows about what is behind the target, ie. the casting player might not be able to create a believable see-thru effect for an object whose other side they cannot conceive themselves. I preferred to leave these explanations out from the spell's description, as I am interested in whether the spell could be balanced against the core rules. (Thanks to @Molot and @Szega for asking about and making this point clearer.)
• I hope it is clear that "down to 11" means the DC can be any number between 11 and your normal spell save DC.

Answer 1

It is balanced, but seems costly

Level

As it is, the spell is really close to being a combination of invisibility and major image. We already have a spell like that, mislead, that is also 5th level. There might be debate about the compared utility of mislead and virtual illusion, but overall the level is adequate. The upcasting effect is also not unusual.

Cost

Another spell that is 5th level and costs 500gp of gems is raise dead, an intentionally costly spell. I believe that your spell should be cheaper, not requiring a costly component, but at the least not consume the gemstone. The aforementioned mislead is free (has only somatic components).

Access

I would recommend giving access to the spell to bards too, as they have access to most illusions.

Notes on the text

• There is no such thing as a passive check. Creatures have passive scores that usually serve as DC-s to other checks. It is not clear whether you mean "the DM should make a check in secret" or "compare passive score to DC". Overall, as no other illusion has a mechanic like this, I would recommend just omitting it and making any observer suspicious.

• There is also no such thing as a standard action, although here it is fairly clear you mean an Action. Allowing a creature to use a bonus action to investigate would practically give away the game, as no other illusion spell allows that.

• Just seeing a casting does not allow anyone to recognize it, they need to spend their reaction to do so. I would not recommend on waiving the requirement to spend the reaction, for the above mentioned reasons.

Answer 2

Overpowered.

Casting this at level 8 means everyone, forever has to spend at least 2 actions in order to target you successfully (or know you are a valid target at least). Given that certain illusions, such as that of the trickery domain cleric, can actually cast spells nothing you do is going to give the game away.

Also pointless.

There is no ability other than true sight in 5e that automatically sees through illusions that this will fool. The default reaction to an illusion is to attack it, not to assume it is an illusion and ignore it, so this is a solution for a problem that doesn't exist.

Answer 3

I think this started with a neat concept. There are many uses, in and out of combat, for making a large amount of creatures suddenly suspicious of a creature or object.

The first issue comes with using a passive DC as a part of the spell. It’s sure a unique effect, but definitely needs to be better worded. A DM wouldn’t even “Make” such a check, since if you used it how I interpret it, as you compare it to the spell save DC, describing it as an active thing the DM has to do makes things even more confusing.

Of course, there’s an inherent problem to making a spell’s effects nearly unavoidable (since few things, other than temporary stat debuffs, which few creatures would/could even inflict on themselves, could reduce a creature’s passive Perception DC), which I’m guessing is the reason you dumped the huge front end cost of a 500gp gem. For the sake of argument, I can say that those two things balance out.

The next problem is right in the next line, lowering your own spell’s DC, by: "purposefully leaving more fake clues, in order to lure creatures more easily.” This sounds like something that would take as either a reaction, to do it as a creature inspects your “illusion”, or a bonus action, to change your DC until your next turn. The other problem with this mechanic, on top of being the second new mechanic in the same paragraph, is what happens when you cast this at a spell slot of 8th level or higher, and don’t need to concentrate on it anymore. I can’t think of the exact consequences that come up if you were able to change the DC even if you weren’t concentrating on it, but it can’t be good from either a role playing or a turn based perspective.

I’ll go over the rest of the little problems just quickly (Mostly cause others mentioned them already):

• Another unique ability letting creatures use a bonus action to inspect the effect gives the spell away immediately
• having two stages of suspicion, while most other illusion spells of this level just need one check to fall for it or see through it.
• Having yet Another check after seeing the “illusion” is a real object, when again, noticing if the object is tangible/intangible usually ends most other illusion spells of this level.
• going back and making the gem defy one of the more basic requirements of spell components and not needing to hold it/have a special place to store it/use a focus.
• feels like it wouldn’t work well against PCs, the spell being obvious due to both the number of checks and clear giveaways for what’s supposed to be a clever and secret spell.

Now I know most of this is just ripping on the spell, but really it can be much more reasonable (and shorter to read) with just a few changes while still keeping the theme:

• Make the Initial viewing a normal perception check, but with a success, the effect takes place (the creature believes the object to be an illusion). If this spell really is intended to be able to fool the perceptive and intelligent with massive perception bonuses, they shouldn’t have any issues beating your spell DC.
• reduce the gem cost or make it not consumed, adding the requirements to at least have it in your hand to start the spell, and (maybe?) remove the disadvantage on an Arcana check, encouraging the caster to use that 120 foot range to be a bit more creative with where they cast the spell from.
• only one check, then let the creature act however they would, believing it to be an illusion or not.

TLDR: neat idea, though you lost the initial idea in a pile of fancy mechanics. Broken/Confusing now, has potential.