I was looking for a way to potentially counter a medusa's petrifying gaze when something hit me: Why not cast the 5th level spell Seeming on her face, giving her a blindfold or something so I can no longer see her eyes? This idea opened up a bit of a can of worms, however.
...If a creature discerns the illusion for what it is, the creature can see through the image, and its other sensory qualities become faint to the creature.
The changes wrought by this spell fail to hold up to physical inspection. For example, if you use this spell to add a hat to a creature's outfit, objects pass through the hat, and anyone who touches it would feel nothing or would feel the creature's head and hair. If you use this spell to appear thinner than you are, the hand of someone who reaches out to touch you would bump into you while it was seemingly still in midair.
A creature can use its action to inspect a target and make an Intelligence (Investigation) check against your spell save DC. If it succeeds, it becomes aware that the target is disguised.
Nowhere does it say that the illusions become "faint" for the viewer or that they can see through them - only that they are aware that the target is disguised. Furthermore, Seeming only fails to hold up to "physical inspection", which the description elaborates as touching and feeling. This leads to a few questions:
What does "becoming aware that the target is disguised" entail? I assume the language for Seeming is different than that of most illusion spells for a specific reason and thus, these illusions do not become necessarily faint, RAW.
If I cast Seeming to put a blindfold on the Medusa's eyes and she fails the Charisma Save, am I immune to the Petrifying Gaze effect? Are others immune to the gaze effect if they choose not to make the Intelligence check?
Is the Medusa able to see through the blindfold? What about after succeeding the Intelligence Check to "become aware of disguises"?
If #3 is a no, this becomes a very powerful spell. Par Seeming's description:
This spell allows you to change the appearance of any number of creatures that you can see within range.
Does the poor wording of this spell then become Mass Blind on a Charisma Save?