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From Rules compendium:

You can ready a standard action, a move action, a swift action, or a free action. To do so, you must specify what you want to do and the conditions under which you will do so. Then, any time before your next turn, you can take the readied action in response to those conditions. The action occurs just before whatever triggers it. If the triggering condition is part of another creature’s activities, you interrupt that creature’s turn. Assuming the interrupted creature is still capable of doing so, it continues its turn once you complete your readied action.

Step by step logical sequence of the round:

To do so, you must specify what you want to do and the conditions under which you will do so.

The condition is a creature starting a movement.

If the triggering condition is part of another creature’s activities, you interrupt that creature’s turn.

The interruption is casting Prismatic wall in front of the moving creature.

Assuming the interrupted creature is still capable of doing so, it continues its turn once you complete your readied action.

The creature MUST continue its turn after the prismatic wall is cast. (because it is capable of movement).

The situation would be: "If the creature is still capable to take the movement it continues its turn after I cast the prismatic wall in front of it." (moving though the prismatic wall).

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This could work but check with your DM

I think this works, but this is not quite as clear cut as you make it out to be.

Can you react to "starting a movement"?

First, there are no specifics what you can use as a condition, so the DM will have to weigh in on that. It seems pretty clear that the condition needs to be observable for your character to react to it. Is it observable that the opponent is "starting movement"? I personally would think this is fine, in particular as there is specific guidance about spellcasting that says:

You can ready an attack against a spellcaster with the trigger "if she starts casting a spell."

If you can react to someone starting to cast a spell, it should be possible to react to someone starting to move, too.

Must the creature carry out the movement?

Second, while it says that the creature continues its turn, it does not say it needs to carry out the turn in exactly the same way. Neither does it say it must continue its turn as you claim, it just says it continues it turn, because the turn is not done yet.

If I want to move somewhere and suddenly a prismatic wall springs up in front of me, I might consider moving somewhere else or do something else entirely instead.

So, rules as written this part does not work. If it would, then you also have to deal with opponents using it against the player characters, and it is quite obvious they would rightfully protest against that. As @Kryan's answer details, nothing in the rules would force the creature to execute its move like that.

But for a dramatic fight, I think this could mostly be fine, especially if you define it as something like "if the creature tries to rush me to attack" or something like that, but you need the DM to go along with it. For example you explicitly can Ready a Weapon against a Charge. However, there the opponent then knowingly charges you, even though you have a spear set ready to pierce them.

There is a cost to readying actions

Casting a prismatic wall offensively would be plenty powerful, and the spell specifically declares its intentions as a defensive device by stating

A prismatic wall spell cast to materialize in a space occupied by a creature is disrupted, and the spell is wasted.

However, there is a hidden cost to this tactic that makes it less powerful than it appears at first glance: to ready, you have to forgo your action, and bet on the other creature moving in on you. If the creature opts to not move at all, and instead casts a spell or uses a breath weapon on you, you just wasted your entire action.

Secondly, depending on how specific your DM wants it, you may even have to declare where exactly you will put your wall, and if they decided to move somewhere else, you have spent a high level spell slot and your action to put an otherwise useless wall onto the battlefield.

In summary, I think this can work, and it can be a cool effect if used once in a climactic battle; but since it would be pretty overpowered if it would work very reliably, and that will make the game less fun, any DM worth their salt will start to nerf this tactic pretty quickly if you want to use it repeatedly, and they have a number of legitimate tools to do so.

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    \$\begingroup\$ I like this answer and I am the kind of GM that would allow it but this is also the time when any NPC with that spell can do the same to players :) \$\endgroup\$ Dec 21, 2023 at 13:01
  • \$\begingroup\$ @justhalf That’s definitely not true. You ready an action to cast a spell on your turn, but you don’t actually cast it until it’s triggered. Otherwise counterspelling wouldn’t work (even more so than it already doesn’t). \$\endgroup\$
    – KRyan
    Dec 22, 2023 at 7:42
  • \$\begingroup\$ @KRyan yea, I noticed the tag 3.5e after I wrote the comment. My apologies. In 5e, readying a spell uses the spell slot (it's reasonable, since normally a spell has a casting time of one action, while in a reaction -- when a readied action triggers -- there is only a very brief moment, so you need to do bulk of the work during the ready action, including the spell slot) \$\endgroup\$
    – justhalf
    Dec 22, 2023 at 8:04
  • \$\begingroup\$ I would allow this trick to work, but with the caveat that I would give the enemy a reflex save vs the spell's save DC (potentially with circumstance modifier based on how fast or slow they were moving) to abort their movement before hitting the wall. \$\endgroup\$
    – Douglas
    Dec 22, 2023 at 18:43
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The creature could just not continue into it

Nobody the Hobgoblin’s fine answer addresses all the considerations here, and gets everything just about right—except that they do not have a definitive answer to “Must the creature carry out the movement?”

The answer to that is “No.” This is, quite simply, because nothing in the game says otherwise. Nothing says that a character taking a move action lays out a predefined path from their starting point to their end point, and is then committed to following that exact path. Characters encountering unseen obstacles or hazards and adjusting their path—or even not having a path, but moving to a point to see what they can see from there and making decisions about where to go after they have that information—are ubiquitous in the game. The only way this strategem would work is if none of those things did—and they do.

There are some actions that do require laying out a predefined path: charging and running. However, even these cases allow you to abandon the planned maneuver (sacrificing the benefits it would have given you). Again, it is the simple case that nothing says otherwise: the rules don’t say “when you decide you are going to charge, you do these things,” it says “if you do these things, you can charge.” If something comes up that causes that plan to go awry, the rules say, you simply “can’t charge.” Nothing in here says you have to.

Ultimately, movement is “actually” done square-by-square. Laying out multiple squares in a path is just a way of speeding up the game, because most of the time you know what you want to do and nothing happens that changes your mind. But at the end of the day, it’s just a shortcut to evaluating your turn, and it’s a shortcut that sometimes requires that you stop, go back, and reconsider because you encountered things you didn’t expect along the way.

So if you did this, the enemy would just stop or turn and not move into the prismatic wall. You might consider the Invisible Spell feat from Cityscape for an option that might actually work, though, if the target does not have the ability to see invisible things or recognize the spell you’re casting (Races of Stone lets a Sleight of Hand check cover the latter).

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  • \$\begingroup\$ This is how it works, so maybe my answer isn't clear enough about this. Nothing forces a creature to move into an obstacle that appears in front of it. What I mean is that if a creature charges you, and it is obvious it will move in your direction (maybe because its primary way of attacking is melee) and all of you are standing in that direction, and the caster states "if it tries to move into this square, I will cast the wall on this edge of the square", the DM can decide they have too much momentum, and run into the wall; but they also can dedice they'll just stop. \$\endgroup\$ Dec 22, 2023 at 6:45
  • \$\begingroup\$ @NobodytheHobgoblin Insofar as rule 0 exists, yes, the DM might decide that. But that’s a houserule; the official rules would not support that at all (rule 0 aside). \$\endgroup\$
    – KRyan
    Dec 22, 2023 at 7:40
  • \$\begingroup\$ I added a bit of language to clarify. \$\endgroup\$ Dec 22, 2023 at 7:59
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    \$\begingroup\$ @NobodytheHobgoblin I appreciate KRyan taking on this question and answering it like I would've. :-) I've written a lot about the ready action like here and here if you really want to go down the rabbit hole, but the short version is if the DM says, "Your momentum sends you careening into the prismatic wall," I'm leaving that DM's campaign, and I'd recommend you do the same. \$\endgroup\$ Dec 22, 2023 at 12:25
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    \$\begingroup\$ @NobodytheHobgoblin I don't want to belabor the point, but if a DM took the narrative liberty of describing a creature's momentum making it move in a way that it no longer wanted to move, I'd be paranoid about the DM's later mechanical rulings on the subject. The DM narrating that happening would sound to me like a warning. I have run an RPG with plotted movement (i.e. Hero System), and while it may (pro tip: does) lead to hilarity, it's deeply antithetical to this game. \$\endgroup\$ Dec 22, 2023 at 15:21

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