Nystul's/Arcanist's Magic Aura is poorly worded and either barely works at all or is potentially catastrophic in its uses.
2nd-level illusion
Casting Time: 1 action
Range: Touch
Components: V, S, M (a small square of silk)
Duration: 24 hours
You place an illusion on a creature or an object you touch so that divination spells reveal false information about it. The target can be a willing creature or an object that isn’t being carried or worn by another creature.
When you cast the spell, choose one or both of the following effects. The effect lasts for the duration. If you cast this spell on the same creature or object every day for 30 days, placing the same effect on it each time, the illusion lasts until it is dispelled.
False Aura: You change the way the target appears to spells and magical effects, such as Detect Magic, that detect magical auras. You can make a nonmagical object appear magical, a magical object appear nonmagical, or change the object’s magical aura so that it appears to belong to a specific school of magic that you choose. When you use this effect on an object, you can make the false magic apparent to any creature that handles the item.
Mask: You change the way the target appears to spells and magical effects that detect creature types, such as a paladin’s Divine Sense or the trigger of a symbol spell. You choose a creature type and other spells and magical effects treat the target as if it were a creature of that type or of that alignment.
Spell Lists: Wizard
False Magic. As "magical auras" aren't a particularly well-defined term, this seems to pretty much just relate to Detect Magic and letting mundane people see things as being magical (which may or may not be a bit of a shock to them, if it's unclear whether or not normal folks can usually tell if some things are magical or not). While curious, I'm mostly going to be ignoring this aspect of the spell, as I don't feel like doing the usual scam of selling bargain luckstones to people.
Mask. ...okay, so, right away the description of this effect contradicts the very first line of the spell's text. The examples given, Divine Sense and Symbol, are not Divination spells. So that means either it doesn't actually work, or we're dealing with some very bizarre "specific vs. general" ruling wherein the overall spell's description is the general rule (divination spells reveal false information), and this particular effect's description is the specific rule that overrides the general.
But even if we take that second reasoning, we're met with another conundrum. The beginning of the description says "You change the way the target appears to spells and magical effects that detect creature types", which we can interpret as any sort of "Detect (X)" ability. Okay, fine, that's cool. And it's even backed up by the first example being the Paladin's Divine Sense ability, which is definitely a kind of detect (X).
...but the second example given is
"the trigger of a Symbol spell", which is
not a detect (X) ability, and simply "detects" creature types by having them be described in the text of the spell.
When you cast this spell, you inscribe a harmful glyph either on a surface (such as a section of floor, a wall, or a table) or within an object that can be closed to conceal the glyph (such as a book, a scroll, or a treasure chest). If you choose a surface, the glyph can cover an area of the surface no larger than 10 feet in diameter. If you choose an object, that object must remain in its place; if the object is moved more than 10 feet from where you cast this spell, the glyph is broken, and the spell ends without being triggered.
The glyph is nearly invisible, requiring an Intelligence (Investigation) check against your spell save DC to find it.
You decide what triggers the glyph when you cast the spell. For glyphs inscribed on a surface, the most typical triggers include touching or stepping on the glyph, removing another object covering it, approaching within a certain distance of it, or manipulating the object that holds it. For glyphs inscribed within an object, the most common triggers are opening the object, approaching within a certain distance of it, or seeing or reading the glyph.
You can further refine the trigger so the spell is activated only under certain circumstances or according to a creature's physical characteristics (such as height or weight), or physical kind (for example, the ward could be set to affect hags or shapechangers). You can also specify creatures that don't trigger the glyph, such as those who say a certain password.
This is... Mildly problematic, given that it means any fiend or undead who has access to someone who can cast this spell can, over the course of a month, become permanently immune to any Symbol or presumably also Magic Circle spells that target that type of creature. It becomes even more problematic since the term "magical effects" gets included, meaning we need to have
that discussion all over again to decide if a Paladin's smite is considered magical, or an Oathbreaker's aura that grants +CHA to damage rolls for nearby friendly "fiends" and "undead".
...it becomes
catastrophically problematic when you realize that spell
targeting is described in a spell's body text. One could argue that "target" and "detect" are distinct classes in the descriptive language, with one being active and the other passive, but... The wording in some spell texts makes this a little blurry, and it basically just comes down to "common sense" fiat.
But if not, and we take the wording "You choose a creature type and other spells and magical effects treat the target as if it were a creature of that type or of that alignment." at face value... Well gee. "Humanoid" is a creature type, and a buttload of spells target "humanoid" specifically. Change your type and never be affected by those spells again.
And this just gets increasingly stupid when you realize that the targeting text doesn't seem to distinguish between "creature" and "object", and the Mask effect
specifically makes the target appear as a given creature type ("or alignment", but the effect text doesn't actually say anything about letting you change its perceived alignment. Which is weird). Think there's a magical trap somewhere? This rock is now a "humanoid", toss it into an area to see if it triggers on humanoids entering its zone.
...or we can crank the stupidity up even more, by virtue of the fact that there are no size restrictions given for a target object; only that it "isn't being carried or worn by another creature". Touch a really big rock. It's now a "beast". Awaken it or do whatever the hell else you'd like to do with a spell that specifically targets a beast/creature.
Interestingly, this can also let you do your very own Weekend at Bernie's cosplay by making a corpse (which is officially an object) be treated like a creature.
Do I think the spell
should be run like that? God no, that's horrifying. But it makes it clear that this is one of those spells that really did not get many whacks with the sanity stick when it was written.
As an aside, Symbol: Death has the potential to deal 1,000d10 damage if you can get it to trigger in a way that the target(s) can't get out of the 60' radius sphere within 10 minutes. Save for half, of course.