Are illusions automatically revealed if someone knows I am an illusionist?

Question

If NPCs know that a character frequently uses illusions, such as they have seen them cast some before, would the illusion automatically fade? If it was copying another spell's effect, like darkness, would the ambiguity stop them from knowing for certain and require a check?

Answer 1

Definitely not.

Merely knowing that someone is an illusionist does not reveal their illusions. Players and NPCs might become suspicious and doubtful if they discover that an illusionist is around, and they might check everything more carefully, but they don't automatically know what's an illusion and what's not.

Each illusion spell contains specific criteria that break the illusion, and they are different in each case. For example, Major Image is broken upon physical interaction:

Physical interaction with the image reveals it to be an Illusion, because things can pass through it.

Whereas Phantasmal Force is not:

While a target is affected by the spell, the target treats the phantasm as if it were real. The target rationalizes any illogical outcomes from interacting with the phantasm.

This means we have to look at the specifics of each spell to see when they are revealed. No illusion spell states that the illusion is broken simply by knowing that there is an illusionist around, so that is not a way that illusions can be revealed. Remember, spells only do what they say they do, and nothing more.

For more evidence, we can look at AngryGM's great post about illusionist encounters, excerpted here:

Let me give you a simple example. Imagine you’re exploring the Castle of Illusions. In your exploration, you encounter some pits that are covered with illusions. Step on the floor, it turns out it was an illusion and you fall. You encounter some other pits that are illusions themselves. Step on the pit, you don’t fall, it’s an illusion. Imagine other pits are illusions that cover pressure plates in the floor that spring other traps. And other pits are just normal, visible pits that work like pits. By the time you’ve run that gauntlet, you don’t know what the hell to think of a pit. Or the floor. You can’t tell what’s real or what’s not. Neither the players nor the characters can. So, then, when they end up fighting the illusionist whose room has several pits in it, they can’t tell which pits are pits and which ones aren’t without experimentation they don’t have time for. And if the illusionist can levitate and stand in a pit OR sometimes just stands on pits that aren’t real, the PCs can’t even tell by his movements which pits are pits.

This scenario sounds like a classic illusionist encounter, right? Some things are illusions, and others are real, but everything is illusions so you can't tell which is which. If illusions were revealed simply because people knew an illusionist was around, this kind of scenario would not work.

Answer 2

No, however the smart (or paranoid) NPC will use their ever present 10' pole to poke every wall and floor section, just like the PC would if you told them there was an enemy illusionist. There is no bonus or modifier just because the character or NPC is suspicious, the same interaction and skill check rules apply, but I would certainly modify NPC actions since he's now wary of illusionary effects. Exactly how paranoid is up to the DM.

Answer 3

It's up to your GM

Using only spell descriptions of illusions like Major Image, there are two ways to learn something is illusionary: trying to interact with it physically and passing a skill check. However, it is reasonable to assume that this list is not meant to be exhaustive. Presumably learning that the effect is illusionary in other ways, such as being told so by the illusionist, would also work, as it allows a creature to "discern" the true nature of the illusion. However, this is also at the GM's discretion, and gets even more complicated if you consider the fact that the illusionist might be lying about something real being an illusion...

However, simply knowing a particular spellcaster uses a lot of illusions is not necessarily enough to discern any particular illusion right away. This would be an appropriate time for a GM to grant advantage to a character trying to discern the illusion, based on their more suspicious stance. This is unfortunately as clear as the rules get - the book is vague on how NPCs and monsters should react to illusions in general.

Answer 4

The point of being an illusionist is not to be able to cast illusory spells that are mistaken for their actual counterpart. These spells being real until the illusion is broken, other agents have to deal with the effects of the spell until or unless it's broken.

And that's when the idea of "illusions stop working when you being an illusionist is known" blows away. There is no need to make it a secret, it may even be the contrary. As said in other answers, your enemies will be way more cautious than necessary if they know you cast illusions. They'll have to check for traps and hidden stuff anywhere, while you won't have to cast a single spell.

Moreover, enemies who believe that your illusions are worthless if they know that you are an illusionist will have a hard time waking up when your illusions will hit them. Of course, they may adopt the reflex of automatically dismiss any illusion you throw at them...

Damned, it seems that fireball was real!

Illusionists aren't limited to illusions and a quick death awaits the fool who assumes so.