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Example scenario:

I ready to cast Booming Blade on a target before they move (either away from me, or stay within my range)

The "before" condition is important, because without it, if the target moves away, the readied spell will be cast after they leave my reach, thus wasting my readied spell. The intent is to have them hit by Booming Blade just before they move, so after that they automatically receive the damage from Booming Blade.

Can I specify the trigger like this?

Do I need to roll perception, initiative, etc. to see if I successfully react before they completely move away from me?

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No - you can't state before as a reaction always occurs after the trigger finishes and the trigger must be perceivable

Rules (emphasis mine):

First, you decide what perceivable circumstance will trigger your reaction.

and

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.

Before they move is not a conceivable circumstance, as it would apply the moment you state your reaction, all of the turns between you and the target, the moment your target uses his action/reaction/bonus action without moving, ... when would your trigger finish? You can't perceive someone being "before they move" and you can't say when the trigger is finished and therefore it's not a valid trigger.

Obviously you can talk to your DM and ask if for example a perception check would allow this trigger, but according to RAW the answer is no.

As T.J.L. points out this is similar to one of the things that the Warcaster feat allows you to do:

When a hostile creature’s movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

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  • \$\begingroup\$ It may also be worth pointing out that the moment you are readying the action could count as "before they move" as they haven't moved yet. a bit pedantic perhaps, but a big part of the issue with readying as "before they move" \$\endgroup\$ – rStyskel Apr 18 '18 at 13:28
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    \$\begingroup\$ @rStyskel That's what I was trying to say when I wrote "Before they move is not a conceivable circumstance, as it would apply the moment you state your reaction". \$\endgroup\$ – Sec SE - clear Monica's name Apr 18 '18 at 13:41
  • \$\begingroup\$ I see that now, I think I became confused where it says you can't perceive someone being before they move (as you can see that they are there and they have not left that space [yet]). \$\endgroup\$ – rStyskel Apr 18 '18 at 15:44
  • \$\begingroup\$ A perceivable circumstance might be "My character thinks they're about to move", but as a DM I would probably add a perception check and a chance of false-positives. \$\endgroup\$ – Mooing Duck Apr 18 '18 at 17:19
  • \$\begingroup\$ It might be worth mentioning that the OP's specific example treads on something that requires a feat (Warcaster) to enable. \$\endgroup\$ – T.J.L. Apr 18 '18 at 19:07
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It depends on the context

Also probably your DM's opinion of the word "perceivable"

A number of other people have quoted the language of the readied action ability, and called specific attention to fact that the trigger for a readied action must be a "perceivable circumstance", and that the earliest your readied action can happen is after the trigger. (The Player's Handbook, pg. 193)

That definitely seems to rule out, "I ready my action, casting booming blade before they take their move action" because how is your character supposed to perceive a move action?

But if you stated it a little bit differently, as "I ready my action, casting booming blade before they run out of my range," things start to get murkier. If you unpack things out to, "I ready my action, casting booming blade when they start to move, before they run out of my range," it gets hard to see why anyone would take issue. The one thing that can get weird is that this kind of trigger can involve the sorts of determinations that usually your DM would call for a skill check or save for (e.g. "Roll dex to see if you can grab your friend before they fall into the inky depths"), but I'm pretty sure that is a product of the rules themselves, not evidence of incorrect play.

The issue here is that the trigger needs to be a "perceivable circumstance". But there are no rules clearly outlining what is or is not a perceivable circumstance in the game of D&D 5e. There are rules for special cases -- your DM may ask you to roll Perception in certain vaguely defined contexts, and there are rules for how you can't hear things when you are in the area of effect of a Silence spell, for example. But there is no mechanical test for, "is x a perceivable circumstance?"

Even worse, there does not seem to be a RAW interpretation for how perceivable circumstances fit into the turn structure of combat, so it is unclear if, for example, your readied action trigger can interrupt (read: happen within/alongside/before) another character's action in the way that an opportunity attack "interrupts the provoking creature’s movement, occurring right before the creature leaves your reach." (Id., pg. 195)

The "perceivable circumstance" language really only tells us to match the triggers to the fiction, not the mechanics. The reason that trying to trigger off of a move action seems pretty obviously wrong based on that language is because we implicitly assume that things like "move actions" and "ability checks" don't really exist in the in-game universe.

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  • \$\begingroup\$ I like that this answer tries to grasp at what was intended behind the words in the manual. In particular, I know I'd DM it as a skill check, and a failure could mean either that the character missed the trigger, or that the character falsely saw the trigger and jumped the gun. I might even give the player some freedom to decide how jumpy they want to be. \$\endgroup\$ – Cort Ammon - Reinstate Monica Apr 18 '18 at 23:36
  • \$\begingroup\$ I like that this answer expands from the impossible "before they move" to discuss the possible "when they start to move" and the cloud of considerations around it. \$\endgroup\$ – Timbo Apr 18 '18 at 23:42
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No, this would not be possible.

The description of the Ready action is as follows:

First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include "If the cultist steps on the trapdoor, I'll pull the lever that opens it," and "If the goblin steps next to me, I move away."

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.

When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration. If your concentration is broken, the spell dissipates without taking effect. For example, if you are concentrating on the web spell and ready magic missile, your web spell ends, and if you take damage before you release magic missile with your reaction, your concentration might be broken.

The Ready action requires a perceptible trigger, and the readied action takes place after that trigger occurs. "Before X occurs" is not a thing you can perceive. In addition, even if it were possible to cast the spell before the target moved, the targeted creature could simply change their mind and decide not to move after suffering the effects of your spell.

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No, but you could ready an action to trigger as they leave your reach:

As the other users have explained, a readied action can only be triggered after a perceivable action occurs. In your specific case, however, if you ready an action to trigger after your target moves, you should be able to use it after they start moving but before they leave your reach. There is already an existing type of reaction that targets an opponent as they leave your reach: An attack of opportunity.

Based on the attack of opportunity, we know that there is enough time to take a reaction between when the opponent starts moving and when they leave your reach. Therefore, there should be no problem triggering your readied action before they're out of range. And since you're giving up your action in addition to your reaction, I don't see any balance issues here.

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