I get the opportunity attack thing and only one reaction in one round but what is a reaction. Could you for example say, “as a reaction to getting hit I hit him back.”?
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Characters (Links!): Amzar Starshade- Odyssey Of Greatness Faelin Nighthollow - 7th Sojourn Dallas Hullbreaker - Legacy of Iggwilv
The game features and elements will define acts as either an Action, Bonus Action, or Reaction. Most acts in the game will be either the Action or Bonus Action; but there are a few features and spells that grant you Reactions.
The best example I can think of that reflects you question, is to take the Ready action and declare that you will "attack a creature if it takes a hostile actions towards you ." So if the creature attacks you or tries to cast as spell at you that will trigger your Reaction to attack. But that requires you to use your Action as your Reaction.
A reaction is a type or action that happens in response to a trigger. To take your example, you would use your action to ready the attack action on your turn. Your trigger would be "If someone hits me, I will hit them back" and that is that. Then you wait and if someone hits you, you can decide to use your readied action or not. If you do it, you use your reaction to try to hit them back. You only get on reaction from the start of your turn until the start of your next turn, so use it carefully :)
Outside of AoO's and the second part of the Ready Action, Reactions are all only available when something calls for them. Bonus Actions are similar: the only general rule that uses them that comes to mind is TWF. As a rule, any action you attempt to improvise uses your, well, Action for the round. Reactions and Bonus Actions can only be used for things that are spelled out in the rules/spells/features/etc.
I think of things like opportunity attacks and the ready action as character features that everyone has. You can 9nly use your reaction for something you have as a feature of your character and only when the trigger for 5hat reaction occurs.
You can only say “as a reaction to getting hit I hit him back.” if you have a feature that allows you to do that. For example if you are a 14th Berserker Barbarian you get.
Retaliation
Starting at 14th level, when you take damage from a creature that is within 5 feet of you, you can use your reaction to make a melee weapon attack against that creature.
So as long as the hit did damage and the attacker is within 5ft you can say that if you are a berserker of 14th level or higher, I am not sure if there are any other examples but most characters will not have this feature so can not.
The other responses described it pretty well. For reference, the rule for reactions is defined in the Combat chapter of the Basic Rules or the PHB:
Reactions
Certain special abilities, spells, and situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's.
What about casting a touch spell (shocking grasp) through a familiar? Does a familiar only acquire a reaction if it gets attacked? It can't even take advantage of an AoO because it can't attack correct? Can an Owl use Flyby to deliver the Shocking Grasp cantrip?
What about casting a touch spell (shocking grasp) through a familiar? Does a familiar only acquire a reaction if it gets attacked? It can't even take advantage of an AoO because it can't attack correct? Can an Owl use Flyby to deliver the Shocking Grasp cantrip?
Familiars can take reactions in general, though they can't make opportunity attacks (because, as you say, they can't attack). A familiar does not "acquire a reaction if it gets attacked". That's...not a thing.
You can cast a spell through the familiar if it has a range of Touch. You cast the spell (usually on your turn) and the familiar uses its reaction to deliver it to a target that it can reach. That's probably the most common use of a familiar's reaction; they generally don't have access to any others.
The Owl's "Flyby" feature lets it avoid provoking opportunity attacks from others when it flies away from them. It doesn't have anything to do with making opportunity attacks or casting spells.
But our DM also states that I can use the familiar to deliver the shocking grasp cantrip, but only if it's reacting to something happening to it.
Just to clarify, we play with the rule that a familiar's turn is the same as it's caster. So our Monk needs to get away from an attacker and since I'm at a safer distance, I want to send my owl down to deliver SG and allow the Monk to get away without an AoO or using their action on Disengage. But can't because nothing is happening to the owl thus allowing it to recieve a Reaction.
But our DM also states that I can use the familiar to deliver the shocking grasp cantrip, but only if it's reacting to something happening to it.
Just to clarify, we play with the rule that a familiar's turn is the same as it's caster. So our Monk needs to get away from an attacker and since I'm at a safer distance, I want to send my owl down to deliver SG and allow the Monk to get away without an AoO or using their action on Disengage. But can't because nothing is happening to the owl thus allowing it to recieve a Reaction.
Is there a different way to word/describe it?
So, your DM has the final say on how anything works at your table, but the idea that the familiar can only deliver the Shocking Grasp spell if it's reacting to something is not in line with the actual published rules. It sounds more like your DM has misunderstood the part of the spell description about the familiar using its reaction to deliver the spell. In the actual rules, what it's reacting to is you casting the spell.
A familiar moving and acting on the same turn as its owner is also a house rule; the published rules say that a familiar rolls its own initiative and acts on its own turn.
In the example you describe, the way it would work in the actual rules is that the owl would need to move to within range of the attacker on its own turn, then on your turn you cast Shocking Grasp and the owl uses its reaction to deliver the spell to the target.
So if the owl has to move in range to touch and then wait for my turn, any turn in between (target) could just take it out?
True, though I think most enemies are unlikely to prioritize attacking an owl when there are more significant threats nearby.
To be clear, I don't think there's anything wrong with having a house rule that lets the familiar act on its owner's turn; that seems fairly reasonable. The other ruling I think is just a mistake on your DM's part because it doesn't make any sense.
In that thread, you'll find the following tweet from Jeremy Crawford, but I don't think it's necessary to wait for the familiar's turn to consume its reaction. It might work also as @wagnarokkrstated here.
I get the opportunity attack thing and only one reaction in one round but what is a reaction. Could you for example say, “as a reaction to getting hit I hit him back.”?
If I ever get into a pbp please make sure you add the underscore to my name when I get PMed.
Characters (Links!):
Amzar Starshade - Odyssey Of Greatness
Faelin Nighthollow - 7th Sojourn
Dallas Hullbreaker - Legacy of Iggwilv
The game features and elements will define acts as either an Action, Bonus Action, or Reaction. Most acts in the game will be either the Action or Bonus Action; but there are a few features and spells that grant you Reactions.
The best example I can think of that reflects you question, is to take the Ready action and declare that you will "attack a creature if it takes a hostile actions towards you ." So if the creature attacks you or tries to cast as spell at you that will trigger your Reaction to attack. But that requires you to use your Action as your Reaction.
A reaction is a type or action that happens in response to a trigger. To take your example, you would use your action to ready the attack action on your turn. Your trigger would be "If someone hits me, I will hit them back" and that is that. Then you wait and if someone hits you, you can decide to use your readied action or not. If you do it, you use your reaction to try to hit them back. You only get on reaction from the start of your turn until the start of your next turn, so use it carefully :)
"Not all those who wander are lost"
Outside of AoO's and the second part of the Ready Action, Reactions are all only available when something calls for them. Bonus Actions are similar: the only general rule that uses them that comes to mind is TWF. As a rule, any action you attempt to improvise uses your, well, Action for the round. Reactions and Bonus Actions can only be used for things that are spelled out in the rules/spells/features/etc.
I think of things like opportunity attacks and the ready action as character features that everyone has. You can 9nly use your reaction for something you have as a feature of your character and only when the trigger for 5hat reaction occurs.
You can only say “as a reaction to getting hit I hit him back.” if you have a feature that allows you to do that. For example if you are a 14th Berserker Barbarian you get.
Retaliation
Starting at 14th level, when you take damage from a creature that is within 5 feet of you, you can use your reaction to make a melee weapon attack against that creature.
So as long as the hit did damage and the attacker is within 5ft you can say that if you are a berserker of 14th level or higher, I am not sure if there are any other examples but most characters will not have this feature so can not.
The other responses described it pretty well. For reference, the rule for reactions is defined in the Combat chapter of the Basic Rules or the PHB:
What about casting a touch spell (shocking grasp) through a familiar? Does a familiar only acquire a reaction if it gets attacked? It can't even take advantage of an AoO because it can't attack correct? Can an Owl use Flyby to deliver the Shocking Grasp cantrip?
Familiars can take reactions in general, though they can't make opportunity attacks (because, as you say, they can't attack). A familiar does not "acquire a reaction if it gets attacked". That's...not a thing.
You can cast a spell through the familiar if it has a range of Touch. You cast the spell (usually on your turn) and the familiar uses its reaction to deliver it to a target that it can reach. That's probably the most common use of a familiar's reaction; they generally don't have access to any others.
The Owl's "Flyby" feature lets it avoid provoking opportunity attacks from others when it flies away from them. It doesn't have anything to do with making opportunity attacks or casting spells.
Understandable on the Flyby.
But our DM also states that I can use the familiar to deliver the shocking grasp cantrip, but only if it's reacting to something happening to it.
Just to clarify, we play with the rule that a familiar's turn is the same as it's caster. So our Monk needs to get away from an attacker and since I'm at a safer distance, I want to send my owl down to deliver SG and allow the Monk to get away without an AoO or using their action on Disengage. But can't because nothing is happening to the owl thus allowing it to recieve a Reaction.
Is there a different way to word/describe it?
So, your DM has the final say on how anything works at your table, but the idea that the familiar can only deliver the Shocking Grasp spell if it's reacting to something is not in line with the actual published rules. It sounds more like your DM has misunderstood the part of the spell description about the familiar using its reaction to deliver the spell. In the actual rules, what it's reacting to is you casting the spell.
A familiar moving and acting on the same turn as its owner is also a house rule; the published rules say that a familiar rolls its own initiative and acts on its own turn.
In the example you describe, the way it would work in the actual rules is that the owl would need to move to within range of the attacker on its own turn, then on your turn you cast Shocking Grasp and the owl uses its reaction to deliver the spell to the target.
So if the owl has to move in range to touch and then wait for my turn, any turn in between (target) could just take it out?
True, though I think most enemies are unlikely to prioritize attacking an owl when there are more significant threats nearby.
To be clear, I don't think there's anything wrong with having a house rule that lets the familiar act on its owner's turn; that seems fairly reasonable. The other ruling I think is just a mistake on your DM's part because it doesn't make any sense.
@tfrance1967 I had a similar doubt in the past but related to Warlocks and Pact of the Chain: Trying pact of the chain for the first time and have questions about attacking with my familiar. - Warlock. Maybe that thread can help you or your DM.
In that thread, you'll find the following tweet from Jeremy Crawford, but I don't think it's necessary to wait for the familiar's turn to consume its reaction. It might work also as @wagnarokkr stated here.