So this is a strange question concerning movement and attacks of opportunity.
When you make an attack of opportunity does the attack and damage happen before the move or after they have left the current position they where in. Main reason I'm asking is if you used booming blade as an AoO would the secondary effect automatically be applied seeing that they would have to move out of your threatening space. So you would have to hit them before they moved out of your weapons reach, and if they got hit and decided not to move to save from the extra damage then you get put into the strange place of them not provoking an AoO.
Yes, they should take the secondary damage from booming blade. They cannot decide to halt their move after an attack of opportunity is resolved, they're already in motion.
You can't use [spell[Booming Blade[/spell] as an Attack of Opportunity as it's not a reaction spell unless they are a War Caster (or there is some other ability I'm not thinking of)?
RAW there is nothing stopping them from pausing their movement after the OoA though?
The attack occurs right before the creature leaves your reach.
The PC can choose to stop movement at any time, so even though they took an OA because they were moving away from you, at the point that Booming Blade is cast on them they can choose to stop moving, so would not immediately take the movement damage from Booming Blade.
I would consider it metagaming for a character to change their movement in response to booming blade, unless they've used their own reaction to identify the spell first.
The attack occurs right before the creature leaves your reach.
The PC can choose to stop movement at any time, so even though they took an OA because they were moving away from you, at the point that Booming Blade is cast on them they can choose to stop moving, so would not immediately take the movement damage from Booming Blade.
This is 100% correct and also the entire point of Booming Blade. It’s to disincentivize movement, not to deal extra damage.
Unless the target has a compelling reason to want to risk the extra damage (which they very well may if they risked the attack of opportunity to begin with), let Booming Blade do what it’s supposed to do: keep the target near the caster.
I would consider it metagaming for a character to change their movement in response to booming blade, unless they've used their own reaction to identify the spell first.
I would consider it metagaming to force someone to continue their movement despite the situation changing. Being sheathed in booming energy is an entirely legitimate reason to stop your movement.
I would consider it metagaming for a character to change their movement in response to booming blade, unless they've used their own reaction to identify the spell first.
I would consider it metagaming to force someone to continue their movement despite the situation changing. Being sheathed in booming energy is an entirely legitimate reason to stop your movement.
Even a non-magical result of an Opportunity Attack could be reason enough to decide not to actually go ahead with the movement that you had started. You could move hoping to avoid the hit or take minimal damage, but then suffer an unexpected critical hit and realise that it is better you not move that way at all actually anymore, and change plan.
Triggering a reaction from another creature does not lock in and force the triggering activity to happen (see being hit and casting shield, or a Sentinel Opportunity Attack, both of which can cancel their triggering activity), but nothing can undo the fact that the trigger has already occurred.
I would consider it metagaming for a character to change their movement in response to booming blade, unless they've used their own reaction to identify the spell first.
Booming Blade is very perceptible. It has somatic components, it deals thunder damage, and it sheathes the target in booming energy, whatever that means. So it's clearly a spell, clearly not a normal melee attack, and then clearly there's some sort of visible condition on the target after you hit. It's totally reasonable for one to stop and consider a new plan after being hit by something like that.
So this is a strange question concerning movement and attacks of opportunity.
When you make an attack of opportunity does the attack and damage happen before the move or after they have left the current position they where in. Main reason I'm asking is if you used booming blade as an AoO would the secondary effect automatically be applied seeing that they would have to move out of your threatening space. So you would have to hit them before they moved out of your weapons reach, and if they got hit and decided not to move to save from the extra damage then you get put into the strange place of them not provoking an AoO.
Booming Blade targets the caster and hence can't be used for an OA even with the War Caster feat.
OAs are a special case of a Reaction that happens before its trigger - you have to make an OA before its triggering movement in general (base rules, no special rules) or your target will be, by definition, out of reach for the attack - but the trigger must still have happened. In other words, OAs rewind time, and they are far from the only rule in the game that does so. Half-Orcs also rewind time, and so does Shield. So the answer to your question is that the creature moves, then you rewind time to make the OA, and then you reset time back to where you started.
To clarify: your question is fundamentally the same question as "if I kill someone with an OA, in which grid square does the corpse drop?", the answer to which is "inside your reach", because the movement was stopped by the OA, just as with the Sentinel feat. The movement that triggers the OA must happen unless the OA itself does something to prevent it.
So if you had a way to legally combine Booming Blade with an OA, the answer to your question would be that the movement had to happen for the OA to happen, so the BB extra damage automatically happens with no way to stop it.
I would consider it metagaming for a character to change their movement in response to booming blade, unless they've used their own reaction to identify the spell first.
This is your prerogative, and the prevailing vibe at a given table might lend itself to that conclusion. Per the written rules, it's allowed, and JC has confirmed in a tweet that it is also intended that way.
But again, I'm not saying you are wrong for calling it metagaming if the situation at your table warrants that judgment.
If you make an Opportunity Attack against a creature that's leaving your reach and the creature's movement isn't impacted in any way by the Opportunity Attack, when the OA is over the creature has taken the 5 feet of movement that caused the OA. The only other possibility would be that the movement never happened so the OA never happened, and the OA by definition happened in this hypothetical.
By the same token, the triggering creature is out the amount of movement it paid to try to leave no matter what happens - once the OA occurs, it's too late to change your mind about spending the movement that made the OA legal.
If you make an Opportunity Attack against a creature that's leaving your reach and the creature's movement isn't impacted in any way by the Opportunity Attack, when the OA is over the creature has taken the 5 feet of movement that caused the OA. The only other possibility would be that the movement never happened so the OA never happened, and the OA by definition happened in this hypothetical.
By the same token, the triggering creature is out the amount of movement it paid to try to leave no matter what happens - once the OA occurs, it's too late to change your mind about spending the movement that made the OA legal.
This is incorrect. Attacks of opportunity explicitly occur prior to their trigger. Sometimes, that means they lead to their trigger not actually happening. The AoO still happens. It’s weird, but it’s not actually that difficult to understand.
It's kind of the same paradox as getting hit, so you cast shield, but then you no longer got hit. Sometimes we just have to accept this is the way it is and not worry about making sense of the interaction of the rules.
So this is a strange question concerning movement and attacks of opportunity.
When you make an attack of opportunity does the attack and damage happen before the move or after they have left the current position they where in. Main reason I'm asking is if you used booming blade as an AoO would the secondary effect automatically be applied seeing that they would have to move out of your threatening space. So you would have to hit them before they moved out of your weapons reach, and if they got hit and decided not to move to save from the extra damage then you get put into the strange place of them not provoking an AoO.
Booming Blade targets the caster and hence can't be used for an OA even with the War Caster feat.
OAs are a special case of a Reaction that happens before its trigger - you have to make an OA before its triggering movement in general (base rules, no special rules) or your target will be, by definition, out of reach for the attack - but the trigger must still have happened. In other words, OAs rewind time, and they are far from the only rule in the game that does so. Half-Orcs also rewind time, and so does Shield. So the answer to your question is that the creature moves, then you rewind time to make the OA, and then you reset time back to where you started.
To clarify: your question is fundamentally the same question as "if I kill someone with an OA, in which grid square does the corpse drop?", the answer to which is "inside your reach", because the movement was stopped by the OA, just as with the Sentinel feat. The movement that triggers the OA must happen unless the OA itself does something to prevent it.
So if you had a way to legally combine Booming Blade with an OA, the answer to your question would be that the movement had to happen for the OA to happen, so the BB extra damage automatically happens with no way to stop it.
FYI, Booming Blade does not target the caster. It has a range of Self but the target remains an enemy within 5 feet of you as specified in the spell's description:
You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you
So this is a strange question concerning movement and attacks of opportunity.
When you make an attack of opportunity does the attack and damage happen before the move or after they have left the current position they where in. Main reason I'm asking is if you used booming blade as an AoO would the secondary effect automatically be applied seeing that they would have to move out of your threatening space. So you would have to hit them before they moved out of your weapons reach, and if they got hit and decided not to move to save from the extra damage then you get put into the strange place of them not provoking an AoO.
Yes, they should take the secondary damage from booming blade. They cannot decide to halt their move after an attack of opportunity is resolved, they're already in motion.
You can't use [spell[Booming Blade[/spell] as an Attack of Opportunity as it's not a reaction spell unless they are a War Caster (or there is some other ability I'm not thinking of)?
RAW there is nothing stopping them from pausing their movement after the OoA though?
The rules say:
The PC can choose to stop movement at any time, so even though they took an OA because they were moving away from you, at the point that Booming Blade is cast on them they can choose to stop moving, so would not immediately take the movement damage from Booming Blade.
This is 100% correct and also the entire point of Booming Blade. It’s to disincentivize movement, not to deal extra damage.
Unless the target has a compelling reason to want to risk the extra damage (which they very well may if they risked the attack of opportunity to begin with), let Booming Blade do what it’s supposed to do: keep the target near the caster.
I would consider it metagaming to force someone to continue their movement despite the situation changing. Being sheathed in booming energy is an entirely legitimate reason to stop your movement.
Even a non-magical result of an Opportunity Attack could be reason enough to decide not to actually go ahead with the movement that you had started. You could move hoping to avoid the hit or take minimal damage, but then suffer an unexpected critical hit and realise that it is better you not move that way at all actually anymore, and change plan.
Triggering a reaction from another creature does not lock in and force the triggering activity to happen (see being hit and casting shield, or a Sentinel Opportunity Attack, both of which can cancel their triggering activity), but nothing can undo the fact that the trigger has already occurred.
Booming Blade is very perceptible. It has somatic components, it deals thunder damage, and it sheathes the target in booming energy, whatever that means. So it's clearly a spell, clearly not a normal melee attack, and then clearly there's some sort of visible condition on the target after you hit. It's totally reasonable for one to stop and consider a new plan after being hit by something like that.
While the target is still within your reach when the Opportunity Attack occur, the rules aren't clear on this particular turn of events. The Dev did suggest an answer on Twitetr https://twitter.com/JeremyECrawford/status/663512273504219136?s=20
This is your prerogative, and the prevailing vibe at a given table might lend itself to that conclusion. Per the written rules, it's allowed, and JC has confirmed in a tweet that it is also intended that way.
But again, I'm not saying you are wrong for calling it metagaming if the situation at your table warrants that judgment.
"Not all those who wander are lost"
If you make an Opportunity Attack against a creature that's leaving your reach and the creature's movement isn't impacted in any way by the Opportunity Attack, when the OA is over the creature has taken the 5 feet of movement that caused the OA. The only other possibility would be that the movement never happened so the OA never happened, and the OA by definition happened in this hypothetical.
By the same token, the triggering creature is out the amount of movement it paid to try to leave no matter what happens - once the OA occurs, it's too late to change your mind about spending the movement that made the OA legal.
This is incorrect. Attacks of opportunity explicitly occur prior to their trigger. Sometimes, that means they lead to their trigger not actually happening. The AoO still happens. It’s weird, but it’s not actually that difficult to understand.
It's kind of the same paradox as getting hit, so you cast shield, but then you no longer got hit. Sometimes we just have to accept this is the way it is and not worry about making sense of the interaction of the rules.
A preaction if you will :)
"Not all those who wander are lost"
FYI, Booming Blade does not target the caster. It has a range of Self but the target remains an enemy within 5 feet of you as specified in the spell's description:
You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you
JC already established this when the question came up post-Tasha's and has been answered as part of Sage Advice -> https://www.sageadvice.eu/the-booming-blade-spell-continues-to-work-with-the-war-caster-feat/
It won't work with Twinned Spell Metamagic anymore though since its range is now "Self" which was probably the main reason they changed this.