Can you move during your reaction in a ready action?

So let’s say that the party or the baddies want to dive a target all at once, but they are far apart on the turn order. Can the divers all do a ready action and then all jump in at once?

Example:

Turn order is:

  1. PC1
  2. PC2
  3. NPC1
  4. PC3
  5. PC4
  6. NPC2

Can NPC1 do a ready action of “when NPC2 moves in to attack a PC, I move in to attack the same PC” and then move and attack as the reaction?

If a spell caster loses concentration on greater invisibility, and an opponent has readied a reaction, what happens first?

As an example, both the spellcaster and the opponent (“the archer”) have 1 hp remaining, so whoever completes there 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, who’s 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 there 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?

Is Ready required on your turn in order to use reaction abilities in between turns?

Do you have to Ready a reaction to use it? Thus, you would have had to used Ready as your action on your turn and wait for the trigger to use the reaction. Would it be correct to say that you cannot use a reaction without first Readying your reaction, except in Opportunity Attacks and abilities that specifically say they don’t have to be Readied? This is in regards to DnD 5e

Can the reaction attack from the third benefit of the Sentinel feat interrupt the triggering attack by the enemy?

The third benefit of the Sentinel feat (PHB, p. 169-170) allows a melee attack when an enemy within 5 feet of you makes an attack against a target other than you (that doesn’t have the feat).

If you kill this enemy with your attack, does their triggering attack still go through?

Same question if say you knock him back or prone, etc.

When does a reaction happen if the trigger is movement?

I realize there are a bunch of these questions about the ready action but I couldn’t find one that answers what I have in mind.

I know by RAW the reaction happens after the trigger finishes but how does that work with movement?

1. What if I specify the distance that must be travelled for the reaction to occur?

For example, if the trigger is “when that creature has moved 10 feet” but the creature’s movement that turn is to be 20 feet without any breaks – do I react at 10 feet or after the intended movement of 20 feet is completed?

2. What if the trigger was only specified as movement by an opponent?

For example, if the trigger is “when that creature moves” but the creature will move a total of 30 feet and plans to break for an attack in the middle of that movement, when do I react? Does the reaction occur after the first 15 feet but before the attack? Does it wait until the whole 30 feet movement is finished?

Even though I’ve used “I” as the player, I am asking as a DM. Is there something in RAW I’m missing or are there any designer comments on this by chance?

Can the reaction attack from the third benefit of the Sentinel feat be triggered by a shove or grapple?

The third benefit of the Sentinel feat says (PHB, p. 169-170; emphasis mine):

  • When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.

Do shoves and grapples meet the requirement of a creature “making an attack” (since they’re described in the rules as “special melee attacks”)?

If so, does the shove or grapple attempt need to be successful in order to trigger the reaction attack from the Sentinel feat?

I suspect that the answer to the first part of my question is yes, given this similar question about the third benefit of the Mobile feat, but I’m less sure about the second part of my question.

(I know that the reaction attack from Sentinel would only occur after the triggering attack by the enemy is resolved.)

How does the reaction timing work for Wrath of the Storm? Can it potentially prevent the damage from the triggering attack?

With attacks of opportunity, the PHB is pretty clear that your reaction occurs as an interruption to your opponent’s move, just before they move beyond your reach, then they resume their turn. Fair enough.

However, there are some abilities that happen during an opponent’s attack – for example, the Shield spell, and a Tempest cleric’s Wrath of the Storm ability. These both say they happen when you’re “hit” by an attack – but the Shield spell raises your AC, so I assume that occurs before rolling damage, and makes it possible for you to be retroactively not hit by the attack. Is the same true for Wrath – that your reaction might KO your opponent, and you wouldn’t have to worry about taking damage?

(I was planning to include Hellish Rebuke in this question but I see it specifically refers to “being damaged”, so I assume that means it happens on your opponent’s turn but definitely after getting hit, meaning you can only cast it if you’re still conscious.)

If a caster readies Time Stop and casts it as a reaction during another creature’s turn, what happens to that creature’s turn?

Imagine Tim and Charlie are in combat. Tim takes the Ready action to cast time stop, with some trigger specified that should occur on Charlie’s turn. Charlie takes his turn, triggers Tim’s readied action in the middle of that turn, and time stop is cast.

What happens to Charlie’s turn? Does it immediately end? Does Tim get to take his turns, and when he’s done, Charlie can continue where he left off?