[5E] Interrupting a Spellcaster via Ready Action

[BluejayJunior]

I can see it going either way and as a player wouldn't really care what the DM decided.
As a DM, I'd probably allow a readied action to force a concentration check to continue casting the spell. If it becomes a usual thing, I can just have the caster not cast a spell or get to a position where they can't see it start casting. There are ways to really make it a choice to give up a full attack action or just risking a single attack that could be wasted if they can't see the enemy start casting.

 

[DND_Reborn]

But finishing the spell need not be the trigger for the Readied Action.

PC Fighter: if the Flayer starts moving its hands or speaking words to cast a spell, I spear it.

Precisely. The condition that triggers the readied action need not mean the action it was part of "finished" as well. That is pretty much up to the DM's discretion/interpretation. Because casting times are so quick in 5E, a readied action is the only way to possibly stop it. It is just one of the ways our group prefers older initiative systems.

Alternatively, you could rule that any attack on the Mind Flayer prior to it going that hits could disrupt the spell before it is cast if he fails a concentration check, but that really only works reasonably well if you don't use the roll-and-repeat standard initiative rules. If you use those, it is great if the player caster rolls high because his spells can't be stopped but sucks if he is last and every attack has a chance of ruining his spell.

In short, there are many ways you can handle this. Find what works best for your group and enjoy!

 

[MarkB]

But finishing the spell need not be the trigger for the Readied Action.

PC Fighter: if the Flayer starts moving its hands or speaking words to cast a spell, I spear it.

That just opens up more room for disagreements. If the Mind Flayer hasn't actually cast the spell yet, how do you know that's what it's doing, as opposed to speaking or gesturing to an ally? Must the Fighter take his readied action the moment the Mind Flayer utters any word or makes any gesture?

And again, you take your readied action after the trigger finishes. What counts as someone finishing starting moving their hands or speaking words?

 

[DND_Reborn]

That just opens up more room for disagreements. If the Mind Flayer hasn't actually cast the spell yet, how do you know that's what it's doing, as opposed to speaking or gesturing to an ally? Must the Fighter take his readied action the moment the Mind Flayer utters any word or makes any gesture?

And again, you take your readied action after the trigger finishes. What counts as someone finishing starting moving their hands or speaking words?

You know he is casting a spell by making an Intelligence (Arcana) check with DC 15 + spell level being cast (XGtE, p. 85). The DM could have told the PC Fighter player, "You see the Mind Flayer start to move its hands and gurgling, roll an arcana check." If the character fails the check, maybe he doesn't recognize the spell as such before the Mind Flayer finishes it? Or the player could just say he throws the spear immediately, risking his throw in case the Mind Flayer was, indeed, faking it.

What I don't like about the lack of casting times in the old sense is it is very hard to stop an enemy caster from using magic unless you are a spellcaster yourself with Counterspell (which Clerics and Druids don't get...).

But I agree it can be open to interpretation so I just let the DM adjudicate it as they see fit.

 

[MarkB]

You know he is casting a spell by making an Intelligence (Arcana) check with DC 15 + spell level being cast (XGtE, p. 85).

Which costs you your reaction, leaving you none with which to take your Readied action.

 

[TheSword]

Which costs you your reaction, leaving you none with which to take your Readied action.

That’s interesting. Is that written somewhere? I’ve not heard that rule but I like it.

 

[DND_Reborn]

Which costs you your reaction, leaving you none with which to take your Readied action.

The rule I stated was for determining the exact spell being cast. The DM could just as easily rule it as Insight check or similar against the Mind Flayer's Deception (if it was actually trying to fool the PC Fighter). Of course, with that thinking the Deception act by the Mind Flayer could constitute its action! LOL

And as I wrote in my post, the player could simple react and throw the spear immediately, without deciding to make the check. Again, lots of ways to handle this.

 

[DND_Reborn]

That’s interesting. Is that written somewhere? I’ve not heard that rule but I like it.

The rule to make the Arcana check does use your reaction. Since the readied action allows you to use your reaction in response to the trigger, if you make the Arcana check you have no reaction remaining to act when the readied action is triggered.

 

[Swarmkeeper]

That just opens up more room for disagreements. If the Mind Flayer hasn't actually cast the spell yet, how do you know that's what it's doing, as opposed to speaking or gesturing to an ally? Must the Fighter take his readied action the moment the Mind Flayer utters any word or makes any gesture?

And again, you take your readied action after the trigger finishes. What counts as someone finishing starting moving their hands or speaking words?

Simple: There should be no disagreement because the DM makes a ruling, like [MENTION=6987520]dnd4vr[/MENTION] stated above. I would say the trigger is finished as soon as the Flayer starts moving its hands or starts to speak in a spell-like fashion, just as the player intended. Then it's up to the player to decide for their PC if they think it is a spell or not. If the player then says their PC tries to recognize the Flayer's motion (or speech) as spellcasting before throwing the spear, the DM might call for an Arcana roll. If the player rolls well, the PC is pretty sure it's a spell (or not). If they roll poorly, they have no idea. In any case, the player gets to decide if the PC carries out the action, or ignores the trigger, per the rules of the Ready action.

How about another example. Let's say the fighter readied this action: "I throw my spear at the Flayer if it starts moving towards us". As a DM, would you really insist the the Flayer finish its movement - perhaps it moves 20' to get next to the wizard, attacks, then moves its last 10' to get next to the fighter - before allowing the fighter to throw the spear? I think this case is very clear: the trigger finishes as soon as the Flayer takes one step closer and the fighter can throw the spear or ignore the trigger, per the rules.

 

[MarkB]

The rule I stated was for determining the exact spell being cast. The DM could just as easily rule it as Insight check or similar against the Mind Flayer's Deception (if it was actually trying to fool the PC Fighter). Of course, with that thinking the Deception act by the Mind Flayer could constitute its action! LOL

And as I wrote in my post, the player could simple react and throw the spear immediately, without deciding to make the check. Again, lots of ways to handle this.

Which still doesn't address the second part of my post. Readied actions, unlike reactions in general, don't occur immediately. They happen after the trigger finishes. That leaves a lot less room for interpretations that allow an action to be interrupted.

That’s interesting. Is that written somewhere? I’ve not heard that rule but I like it.

It's the rule [MENTION=6987520]dnd4vr[/MENTION] was referencing - Xanathar's, page 85.