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As an example, both the spellcaster and the opponent ("the archer") have 1 hp remaining, so whoever completes their action first will knock the opponent unconscious. The spellcaster casts Create Bonfire on the archer's space. However the archer has readied an arrow targeting the spellcaster for "as soon as the spellcaster is visible."

My question is, whose action will go off first?

The relevant information on concentration can be found in the PHB (p.203):

You lose concentration when you cast another spell that requires concentration. You can't concentrate on two spells at once.

and XGtE (p.5):

As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly.

Readied actions can be seen in the PHB (p.193):

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.

From this, it seems unclear who will complete their action first. What counts as the archer's trigger finishing? Does it finish when the spellcaster finishes casting their spell, as this action is what began the trigger? Or does the archer get to fire their arrow as soon as the spellcaster is visible, as they "finish" being invisible as soon as they begin casting the second spell?

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The spellcaster wins

Since invisibility ends as soon as the spellcaster begins casting the archer takes their reaction at the same time as the spellcaster is casting the spell. The archer's readied action and the spellcaster's casting of Bonfire occur simultaneously

Xanathar's Guide to Everything (p. 77) offers the following rule regarding simultaneous effects:

If two or more things happen at the same time on a character or monster's turn, the person at the game table – whether player or GM – who controls that creature [whose turn it is] decides the order in which those things happen.

Since it is the spellcaster's turn they can choose to have the Bonfire be cast before the archer makes their attack. As such the spellcaster wins.

If a DM wanted to be kinder to the archer they could have a Dexterity Check to let the archer shoot faster than the spellcaster finishes casting but this would not be RAW.

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    \$\begingroup\$ Couldn't you make the same argument about counterspell, though? Which would rather defeat the point of it. \$\endgroup\$ – DM_with_secrets May 24 at 6:56
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    \$\begingroup\$ That would be the case except that Counterspell specifies that it interrupts the casting. The specific rule of the spell overrules the general (and optional) rule about simultaneous effects. \$\endgroup\$ – Odo May 24 at 6:58
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    \$\begingroup\$ "If two or more things happen at the same time" -- IMHO it's erroneous to consider this an example of two or more things happening at the same time. Things happen at the same time when they are instantaneous effects and the effects need to be resolved in some order. That's not what's going on here. Rather, there are two instantaneous effects -- the interruption of the invisibility spell and the archer shooting their arrow -- and a longer, non-instantaneous action. A non-instantaneous event can't occur "at the same time" as an instantaneous event, and so this quoted rule is moot. \$\endgroup\$ – Peter Duniho May 24 at 17:46
  • \$\begingroup\$ Let us continue this discussion in chat. \$\endgroup\$ – Odo May 24 at 23:30
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The archer wins

The caster gives up concentration on their first spell the moment they start to focus on a new concentration spell, thus the first ends before the second is complete.

The sequence would be:

  1. The caster is concentrating on Greater Invisibility
  2. The caster drops concentration on Greater Invisibility to cast Create Bonfire
  3. Since the caster is no longer concentrating on their first spell, they become visible
  4. The archer see can now see the caster and fires
    1. Assuming the readied Action was, "As soon as I see the caster, I launch an arrow"
  5. If the archer misses, then the caster can finish their spell uninterrupted

See also this Sage Advice where a readied spell can interrupt another spell caster.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – V2Blast May 25 at 5:37
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The spellcaster wins

With very few exceptions (e.g. Shield, Counterspell), nothing interrupts single (bonus) action spellcasting.

Casting a spell, like making an attack, is an indivisible chunk of action. There is no usable ‘time’ between the start of casting and the end of casting.

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    \$\begingroup\$ Do you have any textual basis to believe that making an attack is indivisible? \$\endgroup\$ – Odo May 23 at 22:53
  • \$\begingroup\$ Wouldn't that make counterspell useless? \$\endgroup\$ – findusl May 25 at 4:40
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    \$\begingroup\$ @findusl no, because specific beats general \$\endgroup\$ – illustro May 26 at 0:22
  • \$\begingroup\$ @illustro I'm specifically concerned with the phrase "Casting a spell, like making an attack, is an indivisible chunk of action.". Because Counterspell requires " you see a creature within 60 feet of you casting a spell". If casting a spell would be invisible, you could not see it. I don't see how "specific beats general" would apply here, as Counterspell does not contain any specific ruling on your being able to see the casting. \$\endgroup\$ – findusl May 26 at 7:39
  • \$\begingroup\$ @findusl Counterspell specifically makes the casting of the spell divisible and interruptible by the specific rules in its spell text. \$\endgroup\$ – illustro May 26 at 8:39
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The archer wins

Assuming they actually hit the spellcaster, that is.

From "Ready" (PHB, 193):

…you can take the Ready action on your turn so that you can act later in the round using your reaction.

(emphasis mine).

So, let's see what a "reaction" does (PHB, 190):

… If the reaction interrupts another creature's turn, that creature can continue its turn right after the reaction.

From these parts of the rules as written, we can see that clearly a reaction can interrupt the turn of another creature. Indeed, we see this in Opportunity Attacks where the reaction (the Opportunity Attack) interrupts the movement of the other creature, which they can then continue after the attack has been resolved.

Likewise, the reaction of the archer, their readied action triggered by the appearance of the spellcaster happens immediately and interrupts whatever else was going on, regardless of whose turn it is.

Since the trigger is not the completion of the spell, but rather the appearance of the spellcaster, and the rules clearly explain that the appearance happens the instant the casting of the new spell begins, and since spellcasting isn't instantaneous, the archer's arrow is loosed before the casting of the spell is complete.

Addendum:

There is clearly a wide range of opinion on this topic. It has been addressed, at least tangentially, in many different forms. See e.g. Timing of the Ready action when the trigger is associated with spell casting?, Can a caster interrupt another caster from casting Time Stop by readying a Time Stop of their own?, Is it possible to interrupt spellcasting?, and Do reactions interrupt their triggers or not?.

There is a certain level of debate even within those questions and answers. Certainly, I would say a majority feel that spell casting cannot be interrupted except by powers which specifically say that it can be (i.e. do not agree with my view). But there is no consensus. More to the point, at least one opposing view has a direct statement from Jeremy Crawford to back that view up.

Quoted one of the answers to one of those many related-if-not-duplicate posts is a tweet from him:

Can silence interrupt a spell caster? I.e. cleric readies silence, trigger is enemy begins spell casting.

As DM, I'd allow a readied silence spell to interrupt the casting of a spell with a verbal component.

I'm obviously biased, but to me this clearly means that casting a spell is something that can be interrupted by a readied action. It's not a long walk from there to determine that any readied action, especially when the trigger is "becomes visible" (which in turn happens the very moment the casting begins), rather than (for example) "casts a spell" (which I readily agree could be interpreted as the entire action of casting), is able to interrupt the casting of a spell, when the trigger is worded appropriately.

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    \$\begingroup\$ There is a slight distinction between interrupting a turn and interrupting an attack/casting a spell. Why do you believe that the cast a spell action, which has already started, would be interrupted by a reaction that can only begin once the spellcasters action has begun? \$\endgroup\$ – Odo May 24 at 5:43
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    \$\begingroup\$ @Odo: it would not be logical otherwise. The rules clearly explain as a point of example that you can interrupt movement. The movement can occur as part of the primary movement allocated for a turn, or as part of taking a Dash action. Either way, opportunity attacks interrupt it. But Dash is clearly an action. Why should one type of action be interruptible, but not another. And a reaction is "an instant response"; there's no additional text (i.e. more specific rule) giving actions the right to be completed before the reaction is carried out. So the general rule holds. \$\endgroup\$ – Peter Duniho May 24 at 16:44
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    \$\begingroup\$ Dash does not include movement as a part of the action. It only doubles the movement available. A person could use their action to Dash and not move at all. \$\endgroup\$ – Odo May 24 at 20:35
  • \$\begingroup\$ Why would we consider casting a spell as an action to not be instantaneous but consider casting a spell as a reaction to be instantaneous? I would suggest that taking a reaction is instantaneous but the things done as a part of that action take the same amount of time as they usually would. \$\endgroup\$ – Odo May 24 at 20:44
  • \$\begingroup\$ @odo: "Why would we consider casting a spell as an action to not be instantaneous but consider casting a spell as a reaction to be instantaneous?" -- I don't know. You tell me. I'm not the one who proposed that was the case. \$\endgroup\$ – Peter Duniho May 25 at 7:06

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