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If a spellcaster is riding a flying creature with flyby (peryton, for example), can the caster deliver a touch spell to a target along the flyby path and then get out without provoking an opportunity attack? Can I use my action in the middle of my mounts flyby action, or does the mount's movement have to be finished before I use my action?

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    \$\begingroup\$ Related Do I draw opportunity attacks when my mount uses flyby? (probably not a duplicate since this linked question is about beast master specifically) \$\endgroup\$ – Sdjz Jan 3 at 17:48
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    \$\begingroup\$ It's related, but not complete. Can I use my action in the middle of my mounts flyby action, or does the mounts movement have to be finished before I use my action? \$\endgroup\$ – KBriggs Jan 3 at 17:56
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Can you attack in the middle of your mount's movement? No.

This was clarified by Jeremy Crawford in his twitter feed a couple of years ago:

Q: rider on controlled mount wants to attack mid-move. Do rider and mount share one turn, or does rider need to Ready attack?

Jeremy Crawford: A rider and a controlled mount have separate turns, but they have the same initiative, which means you decide which one goes first.

So you can't freely act in the middle of your mount's movement; it isn't your turn.

You could ready an action to make your attack when you get close enough, but that adds some limitations:

  • Triggering a ready action is a reaction, so you can't do this and then, say, make an opportunity attack of your own, or cast shield when the guy you just hit decides to retaliate.

  • Readying a spell requires concentration, even if the spell you're holding normally wouldn't. This puts a pretty significant extra limitation on you as a spellcaster: You can't ready a touch spell attack while maintaining a concentration spell at the same time (which of course includes all the good buffs and debuffs).

  • You can't use a bonus action while you're near the target, because your bonus action has to happen on your turn rather than your mount's. Some classes/items/feats/etc and dual-wielding can give you various 'attack as a bonus action' abilities, which couldn't be used during a "ride-by stabbing" like this. A sorcerer's quickened spell metamagic pops to mind as a relevant example.

  • If your class lets you make multiple attacks with an attack action, while afoot you can break up your movement in between individual attack rolls (Player's Handbook p.190); but you can't do that while mounted. If your attack action is a ready action, your triggered action interrupts the mount's movement rather than happening concurrently. (Or to put it another way, because it isn't your turn, you don't have any movement to break up.) And if you stop your mount next to a target and attack during your own turn, you can't really move without getting off your horse first.

Does Flyby protect you from opportunity attacks? Yes.

Here are the relevant rules concepts:

  1. When you are mounted, your mount moves and you are taken with it; you aren't using your own movement.

    [Your mount] moves as you direct it, and it has only three action options; Dash, Disengage, and Dodge. A controlled mount can move and act even on the turn that you mount it. - Player's Handbook, p.198

  2. You only draw opportunity attacks when you use your own movement to leave an enemy's reach.

    You can make an opportunity attack when a hostile creature that you can see moves out of your reach... You also don't provoke an opportunity attack ... when someone or something moves you without using your movement... - Player's Handbook, p.195

Based on rules 1 and 2, you do not draw opportunity attacks while mounted; only your mount does. But...

  1. A special rule relating to Mounted Combat says they can try to hit you anyway.

    If the mount provokes an opportunity attack while you're on it, the attacker can target you or the mount. - Player's Handbook, p.198

That's the key to understanding how mounts and opportunity attacks work. If your mount has an ability (such as Flyby) or uses an action (such as Disengage) that prevents opportunity attacks against it, that also inherently prevents opportunity attacks against you. You don't provoke on your own, and your mount isn't provoking any attacks that could be redirected to you.

Any mount can use Disengage, but Flyby is better because it allows you to use Dodge or Dash at the same time as not provoking attacks.

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  • \$\begingroup\$ OK, so I have to ready the spell on my turn, then take the movement using my mount's turn to trigger the ready condition. Is this practically any different from taking my action in the middle of my mount's turn, ie, is there any practical reason for the DM to make that distinction explicit? \$\endgroup\$ – KBriggs Jan 3 at 19:16
  • \$\begingroup\$ Good point about using the reaction, that's potentially very important (no counterspells that turn, for example). I think this is probably the technically correct answer in that case. I will wait a bit before accepting it to allow a bit more argumentation, I probably jumped the gun accepting the other one. \$\endgroup\$ – KBriggs Jan 3 at 19:35
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    \$\begingroup\$ @KBriggs I'll update my answer to include these caveats and any others I think of. I did come up with one more -- readying a spell requires concentration, which is a very significant limitation for spellcasters. All the good ongoing buff/debuff effects require concentration. \$\endgroup\$ – Darth Pseudonym Jan 3 at 19:51
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    \$\begingroup\$ Could I turn this on its head, and have my mount ready a move action, triggered by my casting of a touch spell? Or can controlled mounts not ready actions? Good call re: concentration, that will certainly affect things in this scenatio \$\endgroup\$ – KBriggs Jan 3 at 19:52
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    \$\begingroup\$ The biggest thing to consider is that a creature (rider or mount) must must complete it's turn before the next character can go. One creature can only act during a different creature's turn via a Reaction. \$\endgroup\$ – T.J.L. Jan 3 at 20:32
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Yes it can, even without the Flyby feature

Can I use my action in the middle of my mounts flyby action

First and foremost, there is no specific "flyby action" the peryton has to take, it just doesn't provoke opportunity attacks. However, the mount itself acts on its own turn (more about it down below).

Now, let's see Monster Manual, page 251:

Flyby. The peryton doesn't provoke an opportunity attack when it flies out of an enemy's reach.

A peryton doesn't provoke opportunity attacks, neither does its rider. See Do I draw opportunity attacks when my mount uses flyby? for more details.

Considering casting a spell in-between — you allowed to do that, but only with your own movement (thanks to Darth Pseudonym for clarifying this):

You can break up your movement on your turn, using some of your speed before and after your action. For example, if you have a speed of 30 feet, you can move 10 feet, take your action, and then move 20 feel.
(PHB 190, "Breaking Up Your Move")

The mount acts on its own turn, so you probably want to Ready a spell beforehand to make this work.

The fun thing is — your mount doesn't even have to have the Flyby feature in order to evade opportunity attacks. Any controlled mount can take the Disengage action:

It moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge.

Therefore, when it isn't busy dashing or dodging, it can disengage and be safe from opportunity attacks for the whole turn:

If you take the Disengage action, your movement doesn't provoke opportunity attacks for the rest of the turn.

That is one of the advantages of being mounted in combat.

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  • \$\begingroup\$ I had not considered the disengage option, interesting. That actually makes flyby rather pointless except in some corner cases like combat between two flying creatures. On a flying mount, you can avoid opportunity attacks just by keeping enough altitude above other creatures in your flight path, and use disengage once for your intended target. \$\endgroup\$ – KBriggs Jan 3 at 18:22
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    \$\begingroup\$ what do you think of the answer below which contradicts this, saying that you cannot split your mount's and your own turn in this manner? \$\endgroup\$ – KBriggs Jan 3 at 19:36
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    \$\begingroup\$ @KBriggs I agree with his reasoning \$\endgroup\$ – enkryptor Jan 3 at 20:17
  • \$\begingroup\$ If the mount is intelligent AND the DM allows you to control it, you can switch to have the mount Ready to move after you make the touch spell/attack. It is perceptible to the mount and should work. \$\endgroup\$ – Vylix Jan 5 at 5:01
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First things first.

If your mount doesn't provoke Opportunity Attacks, neither do you

The mount is using its own movement, not yours, so irrespective of whether the mount is being directly controlled, or if it is being independently controlled, opportunity attacks against you are contingent on whether your mount can be targeted.

So there's nothing wrong with you flying down, delivering a touch spell, and then flying away...

... Unless you're letting your mount act independently. In that case, it's still possible, but you need to modify your Actions.

In this case, what you instead need to do is Ready the touch spell in question, and set the trigger condition to "When my mount brings me adjacent to the creature". Then your mount flies down to them during its own turn, and when it gets adjacent, you use your Reaction to deliver the spell. Then the mount flies away, and because it doesn't provoke Opportunity Attacks, you're not at risk either.

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  • \$\begingroup\$ ...though it's worth noting that the target can similarly ready an action to receive you, regardless of the opportunity attack rules. \$\endgroup\$ – Ben Barden Jan 3 at 18:13
  • \$\begingroup\$ Thanks for pointing out the ready action, another thing I had not considered. \$\endgroup\$ – KBriggs Jan 3 at 18:23

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