Need help at the the table

[TrollCapAmerica]

Question. A PC is about to cast Fireball but takes a opportunity attack. He is arguing that because Fireball is instantaneous he doesn't have his spelling interruped. Can you good folks tell us if he can have the spell intturupted or not

[calebrus]

Question. A PC is about to cast Fireball but takes a opportunity attack. He is arguing that because Fireball is instantaneous he doesn't have his spelling interruped. Can you good folks tell us if he can have the spell intturupted or not

A reaction can only interrupt if it specifically states that it does so, such as Counterspell.
Reactions happen *after* the thing which provokes them (unless otherwise stated).
He gets hit, but he isn't interrupted.

Edit:
And there is no way to provoke an OA by casting. OAs are *ONLY* provoked by movement.
I'm assuming you are referencing Mage Slayer.
He is getting hit with a reaction attack, which is not the same thing, and doesn't interrupt.

[Kane0]

Or a prepared action, which generally happens as a response to the action and happens after the actiom takes place, ie after the fireball is cast the readied action goes off to hit him.

[Gritmonger]

The "time" of fireball is instantaneous, the casting time is one action. And I'd have to ask how other feats like "Sentinel" work then. If the AoO from Sentinel was the result of a foe disengaging, does the above interpretation of "after the action takes place" or "the action takes effect first" mean that the full disengage action and movement would take place before the Sentinel feat kicked in on a successful AoO and reduced the movement to zero? Because it states you get your AoO "before it leaves your reach."

I would say the casting time argument is moot, but the Mage Slayer specifically says it forces disadvantage on concentration checks that require a concentration check. Fireball doesn't require a concentration check. Ergo, the reaction AoO has no effect on the casting of the spell. By itself.

[TrollCapAmerica]

Thanks folks. We went with the "Cast doesn't get interrupted" school of thought after all. Things worked out and the game didn't stall or anything

[jkat718]

@Gritmonger: Reaction attacks are different from Opportunity Attacks. Reactions occur after the trigger occurs, whereas Opportunity Attacks interrupt the trigger. Actually, the text only days that it "interrupts the target's movement," so it's possible that it doesn't interrupt spellcasting. Not sure how to rule that… :smallconfused:

[calebrus]

@Gritmonger: Reaction attacks are different from Opportunity Attacks. Reactions occur after the trigger occurs, whereas Opportunity Attacks interrupt the trigger. Actually, the text only days that it "interrupts the target's movement," so it's possible that it doesn't interrupt spellcasting. Not sure how to rule that… :smallconfused:

It interrupts movement, and then the target is free to continue moving (excluding something like Sentinel).
The reason is simple.
The AO happens after the movement starts.
It only interrupts movement because if it didn't, the target would be out of range for a melee attack by the time the attack occurred. If there were a way around that, it wouldn't even interrupt the movement at all.

But that's irrelevant, because what is occurring is not an OA at all. It's a reaction attack.
All opportunity attacks are reactions. Not all reactions are opportunity attacks.

[jkat718]

@calebrus: OP specified Opportunity Attack, not Gavin attack. I suppose an OA is, technically, a kind if reaction attack. It just specifies that it interrupts movement, overriding the rule that Reactions occur after their trigger. Regardless, you are correct that Mage Slayer specifies a single melee weapon attack as a reaction, and not an OA. The Sentinel feat that Gritmonger mentioned, however, does specify that the attack made is an Opportunity Attack, which therefore interrupts movement. My confusion was that I thought someone was trying to say Sentinel interrupted spellcasting, and I was confused as to why it would.

[calebrus]

@jkat718:
No, the OP says this:
"A PC is about to cast Fireball but takes a opportunity attack."
So one of three things is happening.
1) He is casting and then moving. The OA has no bearing on the spell being cast, because the spell was cast before the movement started.
2) He is moving and then casting. Same as above, but in reverse. The OA has no bearing on the spell being cast, because the OA happens before the spell has begun being cast.
3) He characterized the attack incorrectly. The enemy has Mage Slayer. Mage Slayer doesn't provide an OA, it provides a reaction attack. He simply called it an OA because that's the term he's used to using from 3e.

Number 3 is the most likely. But in no case, at all, under any circumstances, will an OA ever be provoked in a way that interrupts spellcasting. There is one thing that provokes an OA, and that one thing is movement, which is independent of spellcasting.

Combat in 5e is quick and dirty. Gone are the days of making arcana checks to recognize what spells are being cast. Gone are the days where a concentration check is required to cast a spell.
There is no interrupting a spell being cast, except for Counterspell.

[jkat718]

@calebrus: Oh, I see what you mean. Okay, thanks for clearing that up.