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.
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.