Can any actions happen at the same time/instant?

Question

I don't know if this is philosophical or physics...but hopefully we don't have to go there:

Inspired by:
Does the Spirit Guardians spell stack with multiple casters?
How do you determine the 'most potent effect' for overlapping spells?

The effects of the same spell cast multiple times don't combine, however.

So the question I have, is how do you clarify this with the fact that two wizards can cast a fireball (instantaneous) as a readied action at the same time? Do the rules clarify no two "instantaneous" things happen at the same instant, ever, in the whole world?

The only thing I can come up with, is that nothing in the game is supposed to happen at the same time.

This is not indicated for readied actions. So should you also resolve multiple readied actions with initiative rolls and d20 tie breakers?

Note: It seems this would also determine, for instance, a monster affected by an enchantment such that damage breaks the enchantment. I had ruled before that players can surround such a monster, each ready an attack, and each make one attack with advantage before the monster gets a chance. This approach adds realism and removes boring cleanup from the end of combats. However, if the readied actions can't happen at the same time, then it really is the first hit that would wake the monster up, and I am technically breaking rules with this.

Answer 1

Actions are not designed to happen at the same time, rather they happen in discrete chunks.

This is clear from the design of turn based games in general, and especially D&D. This is also apparent, as things that grant reactions usually specify specific timing, either before or after its trigger. Example of the Ready Action (PHB p. 193):

To do so, you can take the Ready action on your turn so that you can act later in the round using your reaction…[w]hen the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.

There are no clearly written rules for resolving multiple readied actions on the same trigger.

However, the easiest, and most straightforward way to resolve this situation is in initiative order for all those readied on the same trigger, and seems to best fit the spirit of the rules, and how they work in general.

An alternative to this, would be to resolve order based on dexterity bonus, however, this introduces additional ties, which should already be resolved by initiative order.

Your last example does let the monster "save" for each attack, but it's still a valid and powerful tactic.

Preventing the monster from responding in between each attack is still a powerful benefit, and may potentially finish it off before it gets another turn.

Being hit while asleep from the Sleep spell, they would wake up on the first strike and lose the unconscious status, and being hit by something that gives a save on damage, such as Tasha's Hideous Laughter, would give them a save for each hit regardless of if all actions happen simultaneously or not, as per the wording of the spells, as each hit is still a separate instance of the target taking damage. Note that victims of Tasha's Hideous Laughter and Sleep would still be prone until they get a turn to stand up.

Finally, the general rule is that things are resolved in initiative order.

There is no specific rule overriding that. Following D&D 5e's system of general rules applying unless a specific one overrides it, I'd conclude this is likely the RAI and RAW approach. See SRD p. 90:

Initiative determines the order of turns during combat. [...] This is the order (called the initiative order) in which they act during each round. The initiative order remains the same from round to round.

Answer 2

Two things cannot happen at the same time.

In the questions you link, the spell effects have a duration, so they can have overlapping durations.

The only case (that I can think of) where two actions might happen at the same time is your example, where two characters ready their actions for the same trigger. Even then, it usually doesn't matter who goes first. For example, in your example with two wizards casting fireball, the two fireballs do not interfere with each other because they have a duration of "instantaneous"; as far as the combat system is concerned, the effects don't take up any time and thus cannot overlap.

Your final example does break RAW

Spells such as sleep specify that the sleeping creature wakes up immediately upon taking damage:

each creature affected by this spell falls unconscious until ... the sleeper takes damage

This is in contrast to spells like Hunger of Hadar:

Any creature that starts its turn in the area takes 2d6 cold damage. Any creature that ends its turn in the area must succeed on a Dexterity saving throw or take 2d6 acid damage as milky, otherworldly tentacles rub against it.

Where the effect takes place at the beginning and/or end of the creature's turn. The difference in wording reflects a clear difference between how these effects are supposed to work. If multiple readied attacks target a sleeping creature, the creature wakes up the moment it takes damage from the first hit, and is not asleep for any attacks that come after. Otherwise, the wording would be "each creature affected by this spell falls unconscious until the turn after the sleeper takes damage".

One could resolve ties using opposed dexterity checks or initiative order, though I personally prefer using dexterity checks, as the two actions may be opposed to one another (two different sides trying to catch a falling object, for example), and it's nice to add some uncertainty into such checks.

As an aside, instead of playing out the boring cleanup at the end of combats, you could simply end the encounter and narrate the final kills--it would have the same effect and save a lot of time (unless, of course, there's some surprise or it's not really the end of the fight).

Answer 3

Everything technically happens at the same time

So basically, everything in combat happens in a round which is 6 seconds. A turn falls within that single round. Initiative determines the sequence of events inside that round.

So the rogue who rolled high on initiative didn't "go first." That rogue had quicker reflexes and was able to strike and quickly skitter away before the enemy could land an effective blow against them (basically, an attack and cunning action disengage).

Meanwhile, the Wizard and Sorcerer who go "next" in the order prepared a fireball each in case a swarm of zombies came through the tunnel door. When the zombies did come through the door, each of the unleashed their prepared fireballs that they set to happen on their initiative count.

So the fireballs would be resolved based on who initially had the higher initiative.

The ogre, seeing this, stomps towards the Wizards but is suddenly gutted by the reaction of a Polearm Master fighter and stopped dead in his tracks.

This all happens in 6 seconds concurrently. So for something like Spirit Guardians overlapping, it's because they last round to round as a concentration spell and only affect targets when they make their saving throw. The way you can look at it is:

Only effects with a duration can occur simultaneously. I have instantaneous effects happen in order of initiative, even when that initiative it shifted to a readied action later. So if the Wizard rolled 18, and the Sorcerer 16, and they both ready an action on the same trigger, the Wizard goes first when it's triggered.

Answer 4

Xanathar's Guide provides a resolution mechanism for things that would happen at the same time

The core rules do not say how to resolve such a situation, so normally it will be up to the DM to decide how to handle it.

There however is an optional rule in Xanathars Guide to Everything (p. 77), if there are multiple simultaneous effects that stack at the same time during a player's turn, the player gets to decide the order in which they happen:

SIMULTANEOUS EFFECTS

Most effects in the game happen in succession, following an order set by the rules or the DM . In rare cases, effects can happen at the same time, especiaUy at the start or end of a creature's turn. 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 DM- who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character's turn, the player decides which of the two effects happens first.

This supports the idea that the way to resolve things that would happen simultaneously, you determine an order in which they happen, and resolve them in that order.

Using this rule, the player who controls the turn where the two wizards want to let go of their readied fireballs decides the order in which they go off.

Answer 5

The rules do not indicate whether readied actions happen at the same time

This leaves you with two choices:

Option 1: Readied actions do not happen at the same time, but in initiative order

• It makes more sense with the reading of rules which indicates actions need to be resolved in initiative order. Note "resolve" in this case means "when it matters," and it is questionable if it matters in these cases.
• The DM probably will have to do a bit less thinking and make less rulings with this approach
• There may be more longer rounds, as you can't just have the PC's all roll at once, and less dramatic things happening
• Other rules do not need to change
• Slightly less realistic and possibly clunky in some cases.

Option 2: Readied actions happen at the same time

• In this scenario, it is possible for two creatures to kill each other
• Players can surround enchanted creatures and attack with advantage or critical simultaneously before it wakes up. Some might consider this more realistic.
• Spells like Tasha's Hideous Laughter become more powerful, particularly when cast on a more powerful single target. Note that these spells are fairly under-powered as written, and it is rare in combat for multiple PC's to have this opportunity. If this one thing bothers you, you can change the wording of the spell descriptions to indicate damage source rather then time of damage, for when they wake.
• To fill a hole in the rules, the DM would have to clarify that instantaneous spells by definition never happen at the same time, to have two fireballs still affect a target.
• Two creatures both trying to pick up an item, for instance, would allow for realistic "struggles," which are supported by the rules, rather then "fastest person gets it."
• Possibly more realistic: You can do things at the same time, like in the real world. There are probably other neat in-game things that become possible as a result.

Note that there is nothing forbidding either approach, therefore they are valid and allowed interpretation of the rules, at least to date. When there is an official ruling on this, both options would remain viable "Variants."

Final Note: What's good for the PC's is good for the enemies. If you go with the Option 2 as a DM, you should look for opportunities for the enemies to use this feature. Few things are "unbalanced," when everyone can do it.