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I'm not entirely sure how to ask this question since this spell starts bending reality in ways that are difficult for me to envision. To be fair, though, that's kind of the point.

This answer posits that the illusion created by the mirage arcane spell is not real and entirely superficial. I'm not sure, but it seems like this runs contrary to the notion that the spell creates tactile elements which makes me think that you could push off the illusion (which isn't unheard of for shadow illusions from 3.5).

The spell's range is "sight" and it can affect "terrain in an area up to 1 square mile", which depending how you shape it, can be incredibly long. Were I to only make it 10' wide, I could run that out for 528 miles (assuming I could see that far away).

My questions:

  • Ignoring the feasibility of seeing that far, could a Wizard use this spell to create the illusion of a smooth path across a stretch of raging ocean to permit the party to walk overland between continents?
  • Do the party members who believe the illusion walk upon the strip of land, enjoying the safety of the land? If so, do they move slower than they think they should? Assume none of them have a swim speed.
  • Or do they fall through the illusion and start drowning? Do they even realize they are drowning? If so, when would they realize that?
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Although I agree with the answer from cpcodes, I think it could be made clearer so I'll make an attempt.

The crux of the question is quite simple: does a "tactile" illusion include the ability to support weight? Clearly a tactile illusion could make metal feel like stone, but if an entire illusory object were tactile could you stand on it? It's possible that this isn't consistent, but Jeremy Crawford, the lead Rules Designer for D&D, has this to say:

You can climb an illusory tree formed by mirage arcane.

While this isn't strictly identical to the proposed use of the spell, it is entirely unambiguous that mirage arcane is capable of producing illusions that support weight, and thus I conclude that an "illusion" of solid ground could be crossed regardless of what the "real" terrain was. All indications I can find are that creatures treat such terrain as if it were real regardless of any ability to "know" that it's not, and thus move at normal speeds.

As for making the affected area into a long path, I don't think that's possible. First of all, the spells I can find with malleable areas are quite explicit about it, but additionally the spell description does not say "one square mile". Instead it says:

an area up to 1 mile square

"1 mile square" more closely follows the phrasing of areas like "20ft square", which I have never seen interpreted as "any shape with an area of 20 square feet", and instead it always means "a square shaped area with sides 20 feet long". This could have been made more explicit (using wording from some other spells, "an area contained within a 1 mile square" would be clearer), but I don't believe you could make a rectangle 2 miles long and 0.5 miles wide let alone a narrow 10 foot path 528 miles long.

You could still cross oceans with it because it isn't a concentration spell, but you will need to either:

  • cast it a lot more than you might have planned
  • be a School of Illusion Wizard (PHB p. 118) of 6th level or above - which would give you access to the Malleable Illusions feature:

    Starting at 6th level, when you cast an illusion spell that has a duration of 1 minute or longer, you can use your action to change the nature of that illusion (using the spell's normal parameters for the illusion), provided that you can see the illusion.

    This feature would allow you to, on getting to the end of your 1-mile illusory land bridge, alter the mirage arcane spell so that the land bridge now starts from where you are, and continues out to sea. Rinse and repeat for as many miles as you want to cross.

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    \$\begingroup\$ "You could still cross oceans with it because it isn't a concentration spell, but you will need to cast it a lot more than you might have planned." It might be worth noting that with Malleable Illusions (6th level Illusionist Wizard feature) you could cross a big ocean by just casting it once - when you get to the end of your illusory ground, change the illusion to start from where you are, etc. etc. \$\endgroup\$ – Vigil Jul 25 '18 at 15:01
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    \$\begingroup\$ @Vigil unfortunately, I only have access to free content from dndbeyond, which does not include the School of Illusion Wizard Tradition, so I can't confidently say how such a feature would interact. Feel free to propose the information in an edit though! \$\endgroup\$ – Kamil Drakari Jul 25 '18 at 15:07
  • \$\begingroup\$ I've proposed that as an edit. :) \$\endgroup\$ – Vigil Jul 25 '18 at 15:30
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The wording of the spell strongly implies that the area affected should be square ("in an area up to 1 mile square"), so I don't think you can reform it in to a strip (or long rectangle). If this was meant, I would think that the wording would be more along the lines of "up to 1 square mile in area which must be contiguous."

The rest of the spell seems to be pretty clear that the spell can make difficult terrain into normal terrain or vice versa, and the last paragraph discussing creatures with True Sight shows that the effect of the spell continues to affect creatures even if they know that it is an illusion.

So:

  1. The wizard could create the illusion of a smooth path across a stretch of raging ocean to permit the party to travel overland between continents, so long as those continents were 1 mile apart or less. Or, more accurately, sqrt(2) miles, since you could rotate the square to place the path between opposite corners. The constant waves, however, might complicate things as your flat land becomes hilly and then back. This part will require DM adjudication.

  2. Those party members that believe the illusion walk upon the land in relative safety as the illusion turns the difficult terrain of the water into non-difficult terrain. They would still have to contend with the weather, as the spell does not change that. One of the examples specifically given is that a "Pond can be made to seem like a grassy meadow", so turning water into land is within the spell's power.

  3. Nobody falls through the illusion unless they try to by digging a hole in the illusory ground or attacking it. Those not trying to fall through the "ground" do not. Those trying to, or those being forced through, would require a DM call - presumably anything that would penetrate a real version of the illusory terrain will similarly affect the illusory version, and it would also be reasonable to assume that it is somewhat more fragile than the real version, but just how much so (if at all) is up to the DM.

    Once below the "ground", they are no longer in the area of the spell (as it only affects the terrain), and so would begin to drown as normal, and be fully aware of it. How they would have to contend with the illusion to get out of the water is again up to the DM. They may have to penetrate the "ground" from the bottom as they did from the top, or they might be able to simply pass through the illusion, only becoming affected by its solidity once fully within the volume affected by the spell, or whatever else the DM decides.

I don't necessarily like the answer you reference, as it ignores the fact that the spell makes the terrain "look, sound, smell, and even feel like some other sort of terrain". The pit example is tricky and makes my head hurt as there is obviously no hole to go into. I would disallow it as part of the illusion since it fundamentally changes the shape of the terrain. Indeed, I feel that the intent of the spell is to only allow a change from difficult terrain to non-difficult terrain, and any further change to the terrain (such as hills or holes) is outside the ability of the spell. If it weren't for the spell specifically allowing the turning of watery terrain into land, i wouldn't have thought it within the spell's capability either. But in the game, water is simple difficult terrain with a chance of drowning. If the spell can alter terrain enough to make a thick bramble into non-damaging, non-difficult terrain, it makes sense that it can make water into non-drowning inducing, non-difficult terrain, though the terrain would have to be flat.

The spell also allows you to turn flat land into swampland, or the opposite of the example above - a grassy meadow into a pond. Which gets us back to the mind bendiness of the pit trap problem. What happens if you decide to go for a swim in the illusory pond. I don't doubt you could drown, in the same way that an illusory brier patch could cause damage. The effect would be similar to other illusions that cause damage (like Phantasmal Force). But since the spell specifically says that it cannot disguise, conceal or add creatures, it bends the mind to figure how a person swimming under imaginary water would appear to others. Fortunately, that's outside the scope of the question and thus this answer, other than to say that it is up to the DM.

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    \$\begingroup\$ For reference, Crawford has ruled on similar questions here: "The mirage arcane spell gives you tremendous latitude in how you make the affected terrain look and feel. The altered terrain can even hurt someone. You could drown in the spell’s illusory lake, for example, or fall off an illusory cliff." And here: "You can climb an illusory tree formed by mirage arcane." \$\endgroup\$ – V2Blast Jul 24 '18 at 23:30
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    \$\begingroup\$ @V2Blast Useful, thanks. Falling off an illusory cliff still seems weird, as it runs into the same problem as the pit in the OP's referenced answer, which is it either implies subtracting real matter to make a hole, or somehow changing the appearance of the person falling (which falls afoul of the "unable to disguise or conceal" clause), unless of course, you add an illusory incline before the cliff, which seems to make the terrain markedly different in geometry, but is less of a violation than the former. In either case, beyond the scope of the question, but handy to have in my back pocket! \$\endgroup\$ – cpcodes Jul 25 '18 at 0:04
  • \$\begingroup\$ @V2Blast I would recommend creating your own answer that makes reference to that tweet. I think it's perhaps the most definitive thing I've seen for the Mirage Arcane spell. \$\endgroup\$ – Pyrotechnical Jul 25 '18 at 1:32
  • \$\begingroup\$ @Pyrotechnical: Thanks for the suggestion! Unfortunately, I don't feel confident enough in my understanding of the spell to formulate an answer to your specific questions from that Sage Advice alone. \$\endgroup\$ – V2Blast Jul 25 '18 at 1:56
  • \$\begingroup\$ I might also add that an illusionist wizard could make the fake terrain real using illusionary reality. \$\endgroup\$ – Garret Gang Jul 25 '18 at 12:45

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