Here is a spell that was initially designed for AD&D 2e. In that edition beings with intelligence scores higher than 18 could immediately recognise illusions of levels up to Int−18. The goal of this spell was to surprise such smart opponents and similar. 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.
I have attempted to update the spell to 5e. 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.
PS: I hope it is clear that “down to 11” means the DC can be any number between 11 and your normal spell save DC.
Zwi’s Virtual 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 it is coming from a few feet away, its 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 a bonus or standard 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. Such a creature falsely discerns the target as an illusion, and can 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, another spellcaster seeing the casting will think that it is major image. 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.