D&D 5E Legal Ready action triggers and order of resolution

[UngeheuerLich]

The trigger for a readied action doesn't have to be an action; it can be an event. The text says "perceivable circumstance", not "action".

So "when the rogue leaves that square" is a trigger. It goes off when the rogue leaves his square. As long as it is a circumstance, and you can perceive to, it is a valid trigger (subject to DM approval, as always).

The example in the book is a goblin stepping on a trapdoor - presumably partway through its move.

I am not sure characters perceive squares as part of the surrounding.
You could however draw a line around the enemy and tell him to not cross it.

 

[morikahn]

Well it clarify you can use a readied action before a spell is cast when the trigger is for casting a spell.

Yeah, you are right: it could be inferred that its possible. I just wish there was a more straightforward answer.

 

[Arial Black]

The trigger for a readied action doesn't have to be an action; it can be an event. The text says "perceivable circumstance", not "action".

So "when the rogue leaves that square" is a trigger. It goes off when the rogue leaves his square. As long as it is a circumstance, and you can perceive to, it is a valid trigger (subject to DM approval, as always).

Morrus is correct. Well, he is 25th level.

The Ready action has two parts: trigger, and (re)action.

The trigger must be a 'perceivable circumstance'. Your character must be able to perceive it. The trigger cannot be something like, 'when he uses a bonus action', because 'actions' are game mechanics that are unknowable to creatures in the game. The trigger must be something that creatures in the game can perceive. So 'when he uses the Attack action' is not a valid trigger, while 'if he tries to belt me with that axe' is!

We know that the process of spellcasting has two parts: 'cause' and 'effect'.

The 'cause' is the process of saying the magic words, wiggling your fingers and holding bat poop; the VSM components. If and when that process completes, then the spell effect comes into existence. Before that process is complete, then no part of the spell can possibly exist yet, because the casting has not been completed yet.

So if you word your trigger like, 'when he casts a spell', you deserve a magic missile to the face! The DM can interpret your wording such that the trigger is 'when the spell has been cast'.

Fortunately, the spellcasting process (VSM components) are perceivable....usually; Subtle Spell means there are no components to perceive, so you're SOL. So you word your trigger like, 'as he begins the verbal, somatic and/or material components of a spell...'. We know that this is allowed in the game, because this is precisely how counterspell is used.

What if you want to trigger if he starts to cast some spells but not others? Well, if you have the spell on your list I would rule that you recognise the spell from those VSM components. If you don't have the spell then I'd ask for an Arcana check, with a DC equal to 10 + the level of the spell being cast. If you are successful, you can make your decision whether or not to actually execute your readied action. If you fail, you still have to make that decision, but you must do so while not being certain what the spell will be.

If you ready an action such that they start to do something but that your readied action resolves before they actually execute theirs, remember that they choose the parameters at the time they execute. If you scatter before the dragon actually breaths, fine. But after everyone has 'moved out of the way', then the dragon is not locked into breathing onto the same exact spot on the landscape; he decides where to aim at the moment he actually breathes. If your are foolish enough to see where he actually breathes before you react, then you get a face full of dragon breath and a valuable lesson about careful wording of triggers.

Ah, but if I react before he actually executes, can he change his mind in response to what I just did?

It depends. If your trigger was VSM components, he is already casting a specific spell so he cannot change it to a different spell half way through. But he decides target, area, effect, all variables at the moment the VSM components are complete and the spell effect begins, so if you run around the corner then he can aim at someone else.

I must admit that I wouldn't allow a readied action to 'interrupt' the casting process of a spell which is cast as either a bonus action or as a reaction, because I believe the VSM casting process to be a single word/gesture, and as soon as you see/hear it then the casting process is complete and the spell effect is already in existence. That's a ruling, not a rule. I think it's a good one though.

 

[Arial Black]

I am not sure characters perceive squares as part of the surrounding.
You could however draw a line around the enemy and tell him to not cross it.

While it is true that creatures cannot perceive the square grid, they can perceive where creatures are and how they move, so I'd take 'when he moves out of that square' as an acceptable trigger because your character can tell if he moves 5 feet or so.

 

[Plaguescarred]

While it is true that creatures cannot perceive the square grid, they can perceive where creatures are and how they move, so I'd take 'when he moves out of that square' as an acceptable trigger because your character can tell if he moves 5 feet or so.

Such trigger narratively could be that i shoot an arrow when the rogue makes a step....

 

[morikahn]

The Ready action has two parts: trigger, and (re)action. [...]

A very good response. Bringing up Counterspell really seals the deal as it would have to interrupt an enemy caster's turn mid-casting and it is a Reaction. So reactions can interrupt an opponent's actions if properly worded.

This really opens up a lot of tactical options to intuitive players and smart NPCs in fights.

 

[Arial Black]

A very good response. Bringing up Counterspell really seals the deal as it would have to interrupt an enemy caster's turn mid-casting and it is a Reaction. So reactions can interrupt an opponent's actions if properly worded.

This really opens up a lot of tactical options to intuitive players and smart NPCs in fights.

If your players are suspicious of your NPC's oh-so-convenient readied actions (Wall of force? Really? You just made that up right now, just to screw us!), get into the habit of writing down the trigger and the (re)action every time an NPC takes a readied action. Only reveal it if the readied action is executed.

 

[morikahn]

If your players are suspicious of your NPC's oh-so-convenient readied actions (Wall of force? Really? You just made that up right now, just to screw us!), get into the habit of writing down the trigger and the (re)action every time an NPC takes a readied action. Only reveal it if the readied action is executed.

Yeah, I was just thinking I'd have to do something like that. Write down the Readied action, fold up the paper, put it out in the open (as it is visible that a spell caster has readied a spell). I'm one of those Dm's that rolls in the open and makes it a point to be very visible and impartial in fights.

I may actually have players do that too just to keep me from being biased.

 

[Maxperson]

Yeah, Cosmic, I'm actually favoring that it is possible to interrupt an action with Ready, as it does seem very reasonable, while the reverse seems unreasonable. It also involves a lot of risk as the character using the Readied action is taking a gamble.

Its opens up a lot of tactics that could potentially cause opponents to waste limited attacks, and I know this could really tick people off in a game.

At my table I would not allow it. I'd allow them to pick the start of an action as a trigger, but since they have not yet begun to take their action, only get ready to take one, they don't suddenly become hasted and able to go before someone else who has already started.

 

[Maxperson]

A very good response. Bringing up Counterspell really seals the deal as it would have to interrupt an enemy caster's turn mid-casting and it is a Reaction. So reactions can interrupt an opponent's actions if properly worded.

Counterspell is a specific rule that trumps general ones. I personally would not view counterspell as something that shows that any reaction can work that way. Spells are often created to work differently than other things can.