Does Phantasmal Force end if the target gains truesight?

Question

Context: If an Illusion wizard casts Phantasmal Force on themselves, can they use the Illusory Reality feature to make things only they can see real?

That question is about what happens when an illusionist wizard uses Illusory Reality to make a Phantasmal Force image into a real thing. The consensus was that this can't be done, because someone under the effect of Phantasmal Force doesn't know it's an illusion. I'm thinking about a workaround for that.

Phantasmal Force, like many illusion spells, allows a creature who can see the illusion to examine it closely, making an Investigation check.

If the check succeeds, the target realizes that the phantasm is an illusion, and the spell ends.

So if the wizard were to inspect the image and discover that it's an illusion, it goes poof. But consider truesight:

A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them...

(Now, if you have truesight then the spell can't be cast on you in the first place. So assume that the illusionist casts Phantasmal Force first, and then her friend casts True Seeing on her.)

The question is: Does truesight (1) provide an alternate way to learn that the illusion isn't real, or (2) cause you to successfully inspect the illusion without having to roll (and maybe without having to use an action)?

If (1), then our illusionist can cast Phantasmal Force and believe the illusion is real, then gain truesight and see that it's not real, and then make it real.

If (2), then truesight will simply break the spell.

There's also a possible (3): Even with truesight, the illusionist will be convinced that the illusion is real. Phantasmal Force has the unusual effect of making the target explain away any inconsistencies, so she might, for example, insist that her truesight doesn't work right. I think this is unlikely (since truesight is described in pretty absolute terms) but maybe there's an argument for it.

(Note that my play style doesn't recognize RAW as a source of authority; I'm concerned with applying the rules in a way that tells a coherent story.)

Answer 1

Truesight does not interact with phantasmal force

Truesight lets you

automatically detect visual illusions

but with phantasmal force

You craft an illusion that takes root in the mind of a creature that you can see within range. (PHB 264, emphasis mine)

A visual illusion is a trick of light, but what this spell creates is a trick of the mind. Having better vision will not help with something that is implanted directly into your head. So the statement of your 3rd case is right, but for slightly different reasons.

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Caveat: This is an interpretation of the rules. Without a setting behind it that explains how illusions work precisely, other interpretations might also be rules-legal.

Answer 2

Thinking an illusion is real doesn't prevent you from using Illusionary Reality on it, so knowing it's an illusion won't help you do so if you otherwise can't

So, Illusionary Reality doesn't require you to think the illusion you choose is an illusion, it just requires it to be an illusion.

If we accept that thinking something isn't an illusion prevents one from using Illusionary Reality on it, as the consensus in the other question proposes, then we are inferring some extra restrictions upon what characters can do around illusions that they don't know are illusions. For example, we might use the same logic to argue that you can't attempt to remove a rock or stick from a region to check if it's subject to Mirage Arcana unless you already know that the region in question is, in fact, so affected. More extreme extensions of this logic might ban checking for illusions via physical interaction without first succeeding at the prescribed Intelligence (investigation) check.

While such reasoning itself leads to the very breakdown of narrative cohesion you seek to avoid, the degree to which it is employed and the extent to which the group is comfortable ignoring the typical rules for illusion spells will inform the outcome of the mechanism you suggest. Ultimately, given that a group operating under such premises has already departed significantly from the rules with regards to illusion magic, the best method to discern the effectiveness of your proposed workaround is to bring it up with the GM and see what they have to say about it. They are ultimately in charge of deciding what works in their games. In general, expect this workaround not to work, because the GM probably has reasons beyond your 'not knowing it is an illusion' to ban the spell interaction if they've decided to do so.

If you are the GM, and your operating procedure is to ignore the RAW in favor of a coherent narrative, you should very much ignore the consensus answer to the interaction between Illusionary Reality and Phantasmal Force and instead rule that Illusionary Reality doesn't work on Phantasmal Force not because of a deeply flawed rules-based argument but rather because it is a balance concern or destabilizing to the game or the game's narrative or whatever your actual reason for having it not work is.

That said, ordinarily the combination does not remove Phantasmal Force, because recognizing Phantasmal Force as an illusion does not automatically end it except via investigation. That leaves either Truesight trumping Phantasmal Force's mind control or vice versa, with no clear RAW preference. While in practice it is more common for higher-level spells to overcome lower-level ones when there are these sorts of conflicts, it's probably more likely that a random group would ignore RAW here and have Phantasmal Force just end when the subject realized it was an illusion by any means, not just the one mentioned in the spell description.