D&D 5E Dispel Magic, Ready action and Fireball

[Arcshot]

Quick question:
My wizard knows his enemy may cast a Fireball but he does not have Counterspell. Can my wizard take a Ready action to cast Dispel Magic instead (ie, enable him to take a reaction and cast a Dispel Magic to null the Fireball the moment the enemy casts it?)

 

[Ilbranteloth]

Quick question:
My wizard knows his enemy may cast a Fireball but he does not have Counterspell. Can my wizard take a Ready action to cast Dispel Magic instead?

Not against an instantaneous spell. You won't know is a fireball until the fireball explodes at which time is to late.

Dispel Magic works against a spell that is currently in effect. So you could ready your action to dispel a spell that has a duration of more than 1 round. If it's a damage causing spell, the target would take the first round of damage.

Ilbranteloth

 

[TheCosmicKid]

Yeah, readying with dispel was how you countered spells in previous editions, but given that counterspell now exists specifically to do this thing I'd be hesitant to allow it in 5E. Counterspell vs. dispel magic should be a meaningful choice for your character -- or you can be extra safe and get both.

 

[Ovinomancer]

By the rules, no, but the opportunity cost for using Dispel is much higher than Counterspell is, in that you specifically have to forgo your action in order to ready, and your trigger may not actually occur. Given that, if a player asked to do it at my table, I would probably allow it, even though it doesn't work according to the rules.

 

[Sage Genesis]

Readied actions are taken "right after the trigger finishes". Since Fireball has an instantaneous duration, your Dispel Magic would be cast after it already happened.

 

[TheCosmicKid]

By the rules, no, but the opportunity cost for using Dispel is much higher than Counterspell is, in that you specifically have to forgo your action in order to ready, and your trigger may not actually occur. Given that, if a player asked to do it at my table, I would probably allow it, even though it doesn't work according to the rules.

Good point.

Okay, let's think about what's actually happening in-universe to see what's plausible. Counterspell obviously works because it has a shorter casting time than most other spells, so your wizard sees a spell being cast and can just speak a few words to disrupt the magic before it's finished. But dispel magic has a casting time about the same as other spells. So how exactly would a wizard use it to counter? [MENTION=6706099]Sage Genesis[/MENTION] mentioned the rule that readied actions go off after their triggers, and I think that rule is actually modeling this in-universe timing: if you start casting (or performing another action) in response to somebody else performing an action, then you're going to finish after they are. You need special, short actions to respond to somebody else's action before it's finished -- i.e., reactions. (If you were to ready a bonus action, it seems reasonable that that would also go off before the triggering action is finished. I'd allow that.)

 

[Ilbranteloth]

Good point.

Okay, let's think about what's actually happening in-universe to see what's plausible. Counterspell obviously works because it has a shorter casting time than most other spells, so your wizard sees a spell being cast and can just speak a few words to disrupt the magic before it's finished. But dispel magic has a casting time about the same as other spells. So how exactly would a wizard use it to counter? @Sage Genesis mentioned the rule that readied actions go off after their triggers, and I think that rule is actually modeling this in-universe timing: if you start casting (or performing another action) in response to somebody else performing an action, then you're going to finish after they are. You need special, short actions to respond to somebody else's action before it's finished -- i.e., reactions. (If you were to ready a bonus action, it seems reasonable that that would also go off before the triggering action is finished. I'd allow that.)

Exactly. Dispel magic isn't designed to counter other spells being cast, no matter what the opportunity cost is. It is to end a spell that is already in effect. Counter spell is specifically designed for this purpose.

Now, having said all this and having had a chance to check my PHB...since they are both 3rd level spells, and there is a higher opportunity cost...it really won't break anything. But the reason they are two different spells is because you'll need to make choices about what to prepare.

If there were rules for interrupting spell casting, and thus possibly ruining the spell, I'd consider other options. But there aren't, and with good reason. There aren't rules for disrupting an attack by a fighter with a longsword, so they've applied the same thing to spell casting.

Preventing spell casting, (or attacking for that matter) by somehow incapacitating them before they cast a spell. Go for it. It requires you to act first.

Ilbranteloth

 

[Ovinomancer]

Good point.

Okay, let's think about what's actually happening in-universe to see what's plausible. Counterspell obviously works because it has a shorter casting time than most other spells, so your wizard sees a spell being cast and can just speak a few words to disrupt the magic before it's finished. But dispel magic has a casting time about the same as other spells. So how exactly would a wizard use it to counter? [MENTION=6706099]Sage Genesis[/MENTION] mentioned the rule that readied actions go off after their triggers, and I think that rule is actually modeling this in-universe timing: if you start casting (or performing another action) in response to somebody else performing an action, then you're going to finish after they are. You need special, short actions to respond to somebody else's action before it's finished -- i.e., reactions. (If you were to ready a bonus action, it seems reasonable that that would also go off before the triggering action is finished. I'd allow that.)

Technically, if you're readying a Dispel, you've already finished casting it and are concentration on holding it for your reaction. So the casting time of a held Dispel is zero -- it's already been cast.

But, as I've already acknowledged, it's not within the rules, so this is clearly a houserule. YMMV and all of that.

 

[spartanah]

I beg to differ
http://www.sageadvice.eu/2015/12/11/can-silence-interrupt-a-spell-caster/
And with this answer I see no difference in dispel magic

 

[TheCosmicKid]

I beg to differ
http://www.sageadvice.eu/2015/12/11/can-silence-interrupt-a-spell-caster/
And with this answer I see no difference in dispel magic

That is a surprising response given the RAW on the timing rules.

There is a difference between silence and dispel magic, though whether it's relevant is up to you: silence is explicitly capable of preventing a spell from being cast, while dispel magic isn't. There are really two distinct questions here: (1) can you time a readied action to interrupt another character's action; and (2) can dispel magic cancel a casting? You can tease these two apart by thinking about whether you could cast dispel magic to stop a spell with a longer casting time, like a ritual.