Is it possible to Ready a spell to be cast just before the start of your next turn by having the trigger be an ally's attack?

Question

Can the trigger criteria for a readied action be as simple as casting after an ally's attack?

If so, are there measures to prevent a pseudo double spell casting like the following example?

Player B: "I would like to ready my action - to cast the Slow spell centered on 'this' enemy, right after Player A makes his attack." (Player A goes right before Player B in initiative.)

Right after Player A makes his attack the Slow Spell is cast.

Player A then ends his turn.

Player B now has a full turn.

Is this a plausible scenario, or does it break the game's mechanics in any way, shape, or form?

Granted, you delay the 1st turn's spell being cast, but you almost guarantee that your spells occur back-to-back without a chance for an end-of-turn save to remove the spell's effects.

Possible issue examples:

• Slow -> disintegrate/fireball.

• Hold Person -> Hex + Scorching Ray at higher levels

Answer 1

This is valid by the rules, but might get called out as metagaming

As Token's answer adequately covers, what you've described is valid according to the rules for readying a spell. However, depending on the style of play used at your table, your DM may disallow it because it is metagaming; that is, having your character act based on out-of-character knowledge. Here's what the rules have to say about rounds and turns in combat (emphasis added):

A typical combat encounter is a clash between two sides, a flurry of weapon swings, feints, parries, footwork, and spellcasting. The game organizes the chaos of combat into a cycle of rounds and turns. A round represents about 6 seconds in the game world. During a round, each participant in a battle takes a turn. The order of turns is determined at the beginning of a combat encounter, when everyone rolls initiative. Once everyone has taken a turn, the fight continues to the next round if neither side has defeated the other.

Note the sentence I have highlighted in bold: within the fiction, characters in the battle aren't really taking turns. They're all fighting simultaneously and continuously, but because simulating real-time combat is not practical, we compromise realism in favor of simplicity and have them all take turns in initiative order. However, this means that the initiative order is part of the game, not part of the game world, and thus the initiative order of combat is out-of-character knowledge, which means using that knowledge to guide your character's actions constitutes metagaming.

So, if you attempt to use this "double-spell combo" at a table that values in-character role playing and frowns on metagaming, you will probably be called out for choosing a trigger condition that has nothing to do with your readied action. You are effectively trying to find a loophole that lets you specify "after the target's turn ends" as your trigger in order to deny the target an end-of-turn save. On the other hand, other tables are happy to embrace the turn-based gameplay and treat combat like a game instead of a simulation, and they will instead congratulate you on the neat trick you found. You probably already have a pretty good idea of which kind of table you're playing at, but if not, you should ask your DM ahead of time before you try to pull out this trick during a session.

Answer 2

Yes, that can be a declared trigger for the ready Action.

The rules for readying that apply here:

Ready, PHB p. 193

First, you decide what perceivable circumstances will trigger your reaction... When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.

This does allow for the pseudo double spellcasting that you mentioned, though this may not be as much of an issue as one might think.

To accomplish this the PC has to take on a few risks and drawbacks:

• He needs to maintain concentration on the spell throughout the entire round; if he loses concentration, he loses the spell.

• It's also possible that the ally (by his own volition or otherwise) might not attack in order to complete the trigger. This would give the PC in question no opportunity to cast his spell.

• Should the PC want to use his reaction for something else, he will have to choose between that and his readied spell. A hefty cost when considering counterspell and other useful reaction spells.

• The PC will also not be able to concentrate on any other spell while he holds his readied spell. Also a hefty cost.

• If the spell cast by the PC had any perceivable casting components, he has then telegraphed his next move. Hostile creatures that understand a spell is being cast may act accordingly. (The PC can't counter the counter!)

This does not break the action economy or constitute an exploit. I can't extrapolate the player's reasoning for doing this, but it does seem to be a rather suboptimal choice to incur all of those negatives instead of an exploit that breaks anything. There is the payoff of being able to disadvantage the creature on its saving throw against his next spell, but, at least to me, it doesn't seem to outweigh all of the opportunity cost involved.