I'm a level 8 eldritch knight with long sword. I've just gained this combo and here is how I see it:
The enemy moves minimum 5ft away from me. This triggers AoO. The attack from the opportunity occurs before the enemy has left my reach.
That means before the enemy has completed moving the minimum 5ft away from me, I can activate war caster and cast BB and roll to hit. On a hit, I roll initial BB dmg. At this point the enemy has completed moving the minimum 5ft away while also becoming "sheathed in booming light" which sets up the second half of the BB potential dmg.
In order for the second half of the BB dmg to proc, the enemy would need to then continue moving (any total movement greater than 5ft).
This tells me that in order to complete the full effects of BB during the AoO, the enemy needs to move a minimum of 10ft away from me in total.
If the enemy chooses not to willingly move for the rest of its turn after the first 5ft, then the second half of BB dmg is basically lost.
Can someone confirm if what I'm saying is correct? Thanks
Since the AoO actually happens before they finish moving away, booming blade's second effect activates while they are 5 feet away, then they choose whether or not to move. It is like a substitute sentinel feat.
I disagree that it is a substitute Sentinel. We all agree that the opportunity attack happens prior to the target actually leaving your reach, but what happens after is not the same.
With Sentinel, the target's speed is reduced to 0 immediately, so they can't move any further; the target is stuck in place still within your reach. This is a specific consequence of reducing a target's movement speed. The target has decided to move out of your reach, and you physically prevent them from completing it.
With Booming Blade, movement is already happening. The attack happens prior to leaving reach, the effect applies prior to leaving reach, the target continues moving out of reach, and the additional damage of BB happens. At this point, the target can decide what it wants to do with the rest of their turn.
I consider this an absolute for a number of reasons, but the key factor is that the targeted creature has already decided that it is going to leave your reach. It has already provoked the attack of opportunity by doing so, and suffers the full consequences of BB as a result. This is a chronological necessity. If we are to say that a creature can choose whether to actually move out of threat range after resolving an attack of opportunity, we are invalidating the provocation of the attack of opportunity itself.
The only thing that can prevent BB from automatically triggering is if there is a secondary consequence (from the AoO itself) that prevents the target from actually leaving your reach. I.e., reducing the target's speed to zero: Sentinel, a couple various class features, special weapon effects, Unconscious, Paralyzed, Stunned, Grappled, Petrified, Restrained, level 5 Exhaustion, death, etc.
Whether the target is aware of Booming Blade or not is irrelevant. It made a decision to leave your reach, and--if it is still capable of moving--it does exactly that. It gets to live or die by the consequences of that decision.
You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
I'm uncomfortable with the result of, "You have decided to move away. I cast the spell and I also melee attack you (as part of the spell) and on top of all of that, you move away to activate even more damage."
I'm uncomfortable with the result of, "You have decided to move away. I cast the spell and I also melee attack you (as part of the spell) and on top of all of that, you move away to activate even more damage."
Why would that make you uncomfortable? The player has made a significant investment in order to be able to do that. I'd be uncomfortable trying to diminish that investment, and the player would certainly be uncomfortable with having their investment diminished for no reason.
Similarly, would you be comfortable with diminishing the Disengage bypassing feature of Sentinel?
DM: "This creature takes the Disengage action, and moves out of your threat range."
Player: "I have the Sentinel feat, so that creature provokes an attack of opportunity from me anyway."
DM: "Then the creature chooses to not move out of your threat range."
Player: "Why? The creature doesn't know that, and you already declared what it is doing. This is what I took the feat for in the first place."
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You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
Since you asked, my discomfort is specific to the behavior of booming blade in the given circumstance. It makes me uncomfortable, because the design of the cantrip is laid out for some damage upfront and some damage later if the target moves. I feel like this circumvents that process and makes it behave in a way it was not intended to. The example you gave with sentinel does not bother me.
Point-blank: War Caster makes all spells cast via the reaction behave in ways they are not normally intended (which is--quasi-ironically--one of the actual intended functions of War Caster to begin with). Booming Blade is not unique in this regard, nor is it likely to ever occur frequently enough to warrant a real revision either. If a player has invested a feat (or more if you need to also get the spell from another source) to be able to heavily punish anyone trying to flee from them mid-combat, then that's a valid tactic. That's literally the purpose of AoO's... you turned your back/ran, so now you get punished. "Look at me. I'm your target now."
That you aren't bothered by the idea of stomping on a player's investment in the Sentinel example is... let's just say that--if you did that to me--I would remove myself from your table immediately.
I disagree with your concern, but I do at least understand how/why it makes you uncomfortable.
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You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
The way I would play it at my table (and let's be real here, this is an edge case to begin with) is for a creature to move away and trigger an OA. The caster casts booming blade as a reaction to the movement and they get their melee attack as part of the spell. So now the target creature is 10' away and has heard the caster verbally cast a spell, which may or may not have done damage to the target and then the target gets whacked with a weapon and is then "sheathed in booming energy," which doesn't sound the least bit subtle to me. At that point, the target creature may or may not reassess their decision to move. If they willingly move after that, the second part of booming blade takes effect. If not, then they stay 10' away from the caster sheathed in booming energy in the short term.
PS - I believe you completely misunderstood my position on the sentinel feat.
PS - I believe you completely misunderstood my position on the sentinel feat.
I truly hope so.
As for the targeted creature "reassessing" their decision to move... sure, it might seem reasonable. All of those things happening are certainly not subtle, and--looking at the sequence in a vacuum--I wouldn't want to continue moving either... if I had the time to actually process and analyze what was happening.
All of this is happening in the span of a singular reaction. A fraction of a round, or roughly 1 second. I'm already moving, I've been stabbed in the back, and my body is all tingly. Split-second: what's my first instinct? Keep moving away from the stabby-stick (that I had already decided on anyway), or stop moving entirely while I ponder what the best course of action is?🤔
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You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
The caster casts booming blade as a reaction to the movement and they get their melee attack as part of the spell. So now the target creature is 10' away...
If not, then they stay 10' away from the caster sheathed in booming energy in the short term.
5 feet. Attacks of opportunity occur before the movement. Otherwise the target would always be out of range of the attack it provoked.
The reason I make the distinction is that booming blade says the person must willingly move. In this case, they moved before the spell was cast (since the movement triggered the reaction, then it must have started first), or at least they made the willful decision to move before the spell was cast. Either way, they have not yet willingly moved after the spell effect is on them. Once that happens, I feel they would need to willingly move from that point in order for it to function as described. And that means I need to either leave them in their originating square or else move them to the next available square and play it forward from there..
Again, this is just my interpretation of the way the spell was meant to work. I am not aware of any specific guidance on this situation so we are left to infer intent based on general rules on movement, casting, and opportunity attacks. We do know from Sage Advice that unwilling movement such as by being pushed and movement that does not change one's location, like using an object, swinging as sword, or even standing up at the cost of half of one's movement will not trigger booming blade. Because of that, I think it is reasonable and does not run afoul of any rules to let the target creature make the call on whether to willingly move as soon as possible after the spell was cast. Whether or not they are aware of the spell effect on them is a completely different matter and they may well keep moving without missing a stride completely oblivious to the boom that it will cause. But then again, an aware target creature might not.
If a player at my table were to feel strongly that such a decision should be made after the OA but before the target creature moved at all, then I'm ok with that interpretation as well. It just doesn't feel as natural to me so it isn't my first choice.
Even crazier would be if the target creature, suspecting the war caster might plan some magical shenanigans, readied the casting action for dispel magic to trigger if the war caster cast any spells. Then, I could see a sequence where the movement happens, booming blade happens as a reaction, the melee attack happens as part of booming blade, the dispel magic happens as a reaction to the reaction and dispels the booming energy, and then the target creature is 10 feet away having been targeted by magic and smacked by a weapon, but otherwise normal. A wild second indeed!
@TexasDevin I know dnd doesnt really have a rule for momentum and things like that but imagine someone being running away and having a bolo thrown at them... or they run through a thorn bush etc. If you're standing still you may not be tripped by the bolo or torn at by the thorns but you're running at full speed. You're not stopping, you're going to take the full effect of whatever you came across.
Even though you feel the booming energy, just like you feel the bolo around your ankles or the thorns catching your shirt, you're already moving at full force... like DxJxC said you're moving too fast to even register as being an issue, let alone to actually stop your movement completely (compared to sentinel where say, a bucket of tar is thrown at you and your movement freezes you)..... based on YOUR own momentum, the continued movement is ALSO considered willing.
IF you run away at HALF speed, like specific traps recommend you doing to avoid the hazard (think thrown bearing that would knock someone prone, unleas walking at half speed to consciously avoid i (it says this in the actual description), where you would have the conciousness AND Not Have the momentum causing you to move forward..... but who runs away at half speed in the first place
RAW is very clear, as far as I'm concerned. I mean, RAI is fine, but with RAW being so clear, I don't see where RAI fits into this. Furthermore, I personally trust the creators and writers to have already considered this combination and determined it was fine, so the RAI is purely the intention of the DM wanting to homebrew these mechanics to nerf the effects.
The AoO happens when the target is within 5/10 ft - this is the important bit, I think, and clear per RAW. By the time the target reaches 10/15 ft, which is required in order to make an AoO in the first place (again, important and clear per RAW), the spell is already in effect, the creature willingly moved there, and so takes the secondary damage.
Personally I would rule that the creature has moved and therefore the booming blade damage occurs.
If the creature chooses not to move to avoid the booming blade damage then it isn't moving away to provoke the opportunity attack. This means the attack/spell doesn't happen because no opportunity attack happens because it didn't move to provide the opportunity to attack.
The decision and action to move has already occurred which allows the opportunity attack. So it gets it all, it can't change its mind because it has already done the movement that allows the opportunity attack.
I haven't had this come up, but I think that's how I would rule. Maybe if the creature had proficiency in Arcana I'd do some sort of check to see if they realize what's happening and stop, but otherwise, they keep moving and get boomed.
A reaction is an instant response to a trigger of some kind... (PHB 190) The trigger is movement. The trigger happens first and causes the reaction. The first 5 feet of movement happens before the effect. Or if you prefer, they happen at the same time. But at any rate, there is no way that the reaction happens before the trigger.
A reaction is an instant response to a trigger of some kind... (PHB 190) The trigger is movement. The trigger happens first and causes the reaction. The first 5 feet of movement happens before the effect. Or if you prefer, they happen at the same time. But at any rate, there is no way that the reaction happens before the trigger.
Specific rules (AoO) overrule general rules (Reaction). An Attack of Opportunity is a type of Reaction, so when taking an attack of opportunity, you follow the attack of opportunity mechanics.
AoO specifically states that the attack happens "right before the target leaves your reach" - so when the attack actually hits, when the spell's effects are initiated, the target is within the 5 ft range. As such, when the target gets to 10 ft. away, that is willing movement on the part of the target, so the secondary damage takes place.
If you want to house rule it different because you don't like it for whatever reason, then that's totally fine. RAW is pretty straightforward, though, and this is the rules and game mechanics board.
A reaction is an instant response to a trigger of some kind... (PHB 190) The trigger is movement. The trigger happens first and causes the reaction. The first 5 feet of movement happens before the effect. Or if you prefer, they happen at the same time. But at any rate, there is no way that the reaction happens before the trigger.
I don't think anyone is arguing that. The sequence happening here is: Trigger > Reaction > Resolve Attack & Immediate Effects > Continue Movement > Resolve Ancillary Effects
Trigger happens:
This happens before the creature leaves the threatened square.
It is provoked by beginning to move out of threat range (turning their back).
Reaction:
The attack of opportunity happens before the provoking creature leaves the threatened square.
They are still in reach.
Resolve Attack & Immediate Effects:
Make the attack roll & resolve damage/effects that are immediate (such as Sentinel reducing speed to zero).
Continue Movement:
Did the AoO miss? (Yes) Creature moves out of the threatened square.
Did the AoO hit? (Yes)Did the attacker have Sentinel? (Yes) Creature does not move out of threatened square. Resolve rest of turn as normal.
Did the AoO hit? (Yes)Did the attacker have Sentinel? (No) Creature moves out of threatened square.
Resolve Ancillary Effects:
If hit with BB, resolve damage. The creature has willingly moved and activated the effect. Resolve rest of turn as normal.
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You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
First of all, thank you for pointing that out. I had not considered the wording of an Opportunity Attack as a specific rule to supersede the general rule for a reaction. I try to keep that in mind when discussing rules, but it got away from me this time. Please take another look at the War Caster feat. When you use your reaction to cast a spell, you are not taking an OA at all. Again, RAW is ambiguous on the sequence of the events as we're in an edge case, but I would say that even the melee attack portion of booming blade falls under the category of the "cast a spell action." I don't believe the specific rule for an OA would apply in this case since there is no OA in the traditional sense.
When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack.
With that in mind, I still feel the trigger (movement) happens first or, at worst, simultaneously to the reaction. Lastly, and I say this with full respect to you, it feels a little condescending when you tell me that I can make house rules for rules I don't like. If that was the point of this discussion, it would have been over long ago :)
Heh. And we're back in the argument from that other thread.
Here's my reasoning (though I know no one will be convinced contrary to their pre-formed opinion here.)
The attack of opportunity has to happen before the move, for the simple reason that most melee weapons have a reach of 5 feet. If the attack happens *after* the move, the target is 10 feet away and you miss automatically because they're out of range. Since that's not the case, AOO happens before the move.
So, what is the trigger for the AOO? A PC is a heroic-level combatant, beyond the skills of mere mortals. You're watching the enemy in melee combat. You see their eyes flicker, they shift their weight, realign their feet, and begin to move away rather than attack. (Have you ever fenced? Footwork is BIG.) THAT'S your trigger for the AOO - the initial signal and the beginning of the move.
Now we have this process: Target makes physical movements indicating the start of their move, which leaves them open to an attack of opportunity. PC seizes that opportunity and makes an attack while they're vulnerable, but before they get out of range. Enemy is already committed to the move and has no choice but to take the attack as they move.
Trigger -> attack -> movement.
Warcaster specifically lets you use a spell in place of a simple melee attack. Booming blade is a purely verbal component (plus the weapon you're already swinging at the enemy.)
So, in Matrix-style bullet-time: you observe the trigger, and take your attack of opportunity by swinging your weapon and shouting the verbal components of booming blade. You connect, dealing your weapon damage, and the enemy is *instantly* sheathed in booming energy, which is then triggered as they complete their move.
I don't think anyone is arguing that. The sequence happening here is: Trigger > Reaction > Resolve Attack & Immediate Effects > Continue Movement > Resolve Ancillary Effects
Trigger happens:
This happens before the creature leaves the threatened square.
It is provoked by beginning to move out of threat range (turning their back).
Reaction:
The attack of opportunity happens before the provoking creature leaves the threatened square.
They are still in reach.
Resolve Attack & Immediate Effects:
Make the attack roll & resolve damage/effects that are immediate (such as Sentinel reducing speed to zero).
Continue Movement:
Did the AoO miss? (Yes) Creature moves out of the threatened square.
Did the AoO hit? (Yes)Did the attacker have Sentinel? (Yes) Creature does not move out of threatened square. Resolve rest of turn as normal.
Did the AoO hit? (Yes)Did the attacker have Sentinel? (No) Creature moves out of threatened square.
Resolve Ancillary Effects:
If hit with BB, resolve damage. The creature has willingly moved and activated the effect. Resolve rest of turn as normal.
If this is the case, then I think we are on the same page.
Oh my, I had forgotten about that little tidbit in War Caster. My bad. Sorry.
However, in order to use that mechanic, the conditions for taking an OA have to be there.
Even so, BB has a range of 5 ft, so if BB even works and hits, then the target is within that 5 ft range. When the target gets to 10ft away, the secondary damage is inflicted because they willingly moved.
So, I think there's the argument about specifically where in space the attack takes place - when the target is within 5 ft or 10 ft of the caster. Personally, my interpretation would be that it takes place within the 5 ft. range.
If you wouldn't allow bb to trigger (or more likely allow a PC/monster to stop their movements to avoid triggering bb) then it would a paradox about when triggering the OA in the first place.
Target stepped away more than 5' allowing an OA against him & being hit with bb.
Target noticed bb so stopped moving (stayed within 5').
If target stayed within 5' of attacker it wouldnt have triggered the OA in the first place.... so he shouldn't have been attacked at all..
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I disagree that it is a substitute Sentinel. We all agree that the opportunity attack happens prior to the target actually leaving your reach, but what happens after is not the same.
With Sentinel, the target's speed is reduced to 0 immediately, so they can't move any further; the target is stuck in place still within your reach. This is a specific consequence of reducing a target's movement speed. The target has decided to move out of your reach, and you physically prevent them from completing it.
With Booming Blade, movement is already happening. The attack happens prior to leaving reach, the effect applies prior to leaving reach, the target continues moving out of reach, and the additional damage of BB happens. At this point, the target can decide what it wants to do with the rest of their turn.
I consider this an absolute for a number of reasons, but the key factor is that the targeted creature has already decided that it is going to leave your reach. It has already provoked the attack of opportunity by doing so, and suffers the full consequences of BB as a result. This is a chronological necessity. If we are to say that a creature can choose whether to actually move out of threat range after resolving an attack of opportunity, we are invalidating the provocation of the attack of opportunity itself.
The only thing that can prevent BB from automatically triggering is if there is a secondary consequence (from the AoO itself) that prevents the target from actually leaving your reach. I.e., reducing the target's speed to zero: Sentinel, a couple various class features, special weapon effects, Unconscious, Paralyzed, Stunned, Grappled, Petrified, Restrained, level 5 Exhaustion, death, etc.
Whether the target is aware of Booming Blade or not is irrelevant. It made a decision to leave your reach, and--if it is still capable of moving--it does exactly that. It gets to live or die by the consequences of that decision.
You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
I'm uncomfortable with the result of, "You have decided to move away. I cast the spell and I also melee attack you (as part of the spell) and on top of all of that, you move away to activate even more damage."
"Not all those who wander are lost"
Why would that make you uncomfortable? The player has made a significant investment in order to be able to do that. I'd be uncomfortable trying to diminish that investment, and the player would certainly be uncomfortable with having their investment diminished for no reason.
Similarly, would you be comfortable with diminishing the Disengage bypassing feature of Sentinel?
You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
Since you asked, my discomfort is specific to the behavior of booming blade in the given circumstance. It makes me uncomfortable, because the design of the cantrip is laid out for some damage upfront and some damage later if the target moves. I feel like this circumvents that process and makes it behave in a way it was not intended to. The example you gave with sentinel does not bother me.
"Not all those who wander are lost"
Point-blank: War Caster makes all spells cast via the reaction behave in ways they are not normally intended (which is--quasi-ironically--one of the actual intended functions of War Caster to begin with). Booming Blade is not unique in this regard, nor is it likely to ever occur frequently enough to warrant a real revision either. If a player has invested a feat (or more if you need to also get the spell from another source) to be able to heavily punish anyone trying to flee from them mid-combat, then that's a valid tactic. That's literally the purpose of AoO's... you turned your back/ran, so now you get punished. "Look at me. I'm your target now."
That you aren't bothered by the idea of stomping on a player's investment in the Sentinel example is... let's just say that--if you did that to me--I would remove myself from your table immediately.
I disagree with your concern, but I do at least understand how/why it makes you uncomfortable.
You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
The way I would play it at my table (and let's be real here, this is an edge case to begin with) is for a creature to move away and trigger an OA. The caster casts booming blade as a reaction to the movement and they get their melee attack as part of the spell. So now the target creature is 10' away and has heard the caster verbally cast a spell, which may or may not have done damage to the target and then the target gets whacked with a weapon and is then "sheathed in booming energy," which doesn't sound the least bit subtle to me. At that point, the target creature may or may not reassess their decision to move. If they willingly move after that, the second part of booming blade takes effect. If not, then they stay 10' away from the caster sheathed in booming energy in the short term.
PS - I believe you completely misunderstood my position on the sentinel feat.
"Not all those who wander are lost"
I truly hope so.
As for the targeted creature "reassessing" their decision to move... sure, it might seem reasonable. All of those things happening are certainly not subtle, and--looking at the sequence in a vacuum--I wouldn't want to continue moving either... if I had the time to actually process and analyze what was happening.
All of this is happening in the span of a singular reaction. A fraction of a round, or roughly 1 second. I'm already moving, I've been stabbed in the back, and my body is all tingly. Split-second: what's my first instinct? Keep moving away from the stabby-stick (that I had already decided on anyway), or stop moving entirely while I ponder what the best course of action is?🤔
You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
5 feet. Attacks of opportunity occur before the movement. Otherwise the target would always be out of range of the attack it provoked.
The reason I make the distinction is that booming blade says the person must willingly move. In this case, they moved before the spell was cast (since the movement triggered the reaction, then it must have started first), or at least they made the willful decision to move before the spell was cast. Either way, they have not yet willingly moved after the spell effect is on them. Once that happens, I feel they would need to willingly move from that point in order for it to function as described. And that means I need to either leave them in their originating square or else move them to the next available square and play it forward from there..
Again, this is just my interpretation of the way the spell was meant to work. I am not aware of any specific guidance on this situation so we are left to infer intent based on general rules on movement, casting, and opportunity attacks. We do know from Sage Advice that unwilling movement such as by being pushed and movement that does not change one's location, like using an object, swinging as sword, or even standing up at the cost of half of one's movement will not trigger booming blade. Because of that, I think it is reasonable and does not run afoul of any rules to let the target creature make the call on whether to willingly move as soon as possible after the spell was cast. Whether or not they are aware of the spell effect on them is a completely different matter and they may well keep moving without missing a stride completely oblivious to the boom that it will cause. But then again, an aware target creature might not.
If a player at my table were to feel strongly that such a decision should be made after the OA but before the target creature moved at all, then I'm ok with that interpretation as well. It just doesn't feel as natural to me so it isn't my first choice.
Even crazier would be if the target creature, suspecting the war caster might plan some magical shenanigans, readied the casting action for dispel magic to trigger if the war caster cast any spells. Then, I could see a sequence where the movement happens, booming blade happens as a reaction, the melee attack happens as part of booming blade, the dispel magic happens as a reaction to the reaction and dispels the booming energy, and then the target creature is 10 feet away having been targeted by magic and smacked by a weapon, but otherwise normal. A wild second indeed!
"Not all those who wander are lost"
@TexasDevin I know dnd doesnt really have a rule for momentum and things like that but imagine someone being running away and having a bolo thrown at them... or they run through a thorn bush etc. If you're standing still you may not be tripped by the bolo or torn at by the thorns but you're running at full speed. You're not stopping, you're going to take the full effect of whatever you came across.
Even though you feel the booming energy, just like you feel the bolo around your ankles or the thorns catching your shirt, you're already moving at full force... like DxJxC said you're moving too fast to even register as being an issue, let alone to actually stop your movement completely (compared to sentinel where say, a bucket of tar is thrown at you and your movement freezes you)..... based on YOUR own momentum, the continued movement is ALSO considered willing.
IF you run away at HALF speed, like specific traps recommend you doing to avoid the hazard (think thrown bearing that would knock someone prone, unleas walking at half speed to consciously avoid i (it says this in the actual description), where you would have the conciousness AND Not Have the momentum causing you to move forward..... but who runs away at half speed in the first place
I fully agree with Sigred here.
RAW is very clear, as far as I'm concerned. I mean, RAI is fine, but with RAW being so clear, I don't see where RAI fits into this. Furthermore, I personally trust the creators and writers to have already considered this combination and determined it was fine, so the RAI is purely the intention of the DM wanting to homebrew these mechanics to nerf the effects.
The AoO happens when the target is within 5/10 ft - this is the important bit, I think, and clear per RAW. By the time the target reaches 10/15 ft, which is required in order to make an AoO in the first place (again, important and clear per RAW), the spell is already in effect, the creature willingly moved there, and so takes the secondary damage.
It's really pretty simple. *shrugs*
I haven't had this come up, but I think that's how I would rule. Maybe if the creature had proficiency in Arcana I'd do some sort of check to see if they realize what's happening and stop, but otherwise, they keep moving and get boomed.
A reaction is an instant response to a trigger of some kind... (PHB 190) The trigger is movement. The trigger happens first and causes the reaction. The first 5 feet of movement happens before the effect. Or if you prefer, they happen at the same time. But at any rate, there is no way that the reaction happens before the trigger.
"Not all those who wander are lost"
Specific rules (AoO) overrule general rules (Reaction). An Attack of Opportunity is a type of Reaction, so when taking an attack of opportunity, you follow the attack of opportunity mechanics.
AoO specifically states that the attack happens "right before the target leaves your reach" - so when the attack actually hits, when the spell's effects are initiated, the target is within the 5 ft range. As such, when the target gets to 10 ft. away, that is willing movement on the part of the target, so the secondary damage takes place.
If you want to house rule it different because you don't like it for whatever reason, then that's totally fine. RAW is pretty straightforward, though, and this is the rules and game mechanics board.
I don't think anyone is arguing that. The sequence happening here is: Trigger > Reaction > Resolve Attack & Immediate Effects > Continue Movement > Resolve Ancillary Effects
You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
First of all, thank you for pointing that out. I had not considered the wording of an Opportunity Attack as a specific rule to supersede the general rule for a reaction. I try to keep that in mind when discussing rules, but it got away from me this time. Please take another look at the War Caster feat. When you use your reaction to cast a spell, you are not taking an OA at all. Again, RAW is ambiguous on the sequence of the events as we're in an edge case, but I would say that even the melee attack portion of booming blade falls under the category of the "cast a spell action." I don't believe the specific rule for an OA would apply in this case since there is no OA in the traditional sense.
With that in mind, I still feel the trigger (movement) happens first or, at worst, simultaneously to the reaction. Lastly, and I say this with full respect to you, it feels a little condescending when you tell me that I can make house rules for rules I don't like. If that was the point of this discussion, it would have been over long ago :)
"Not all those who wander are lost"
Heh. And we're back in the argument from that other thread.
Here's my reasoning (though I know no one will be convinced contrary to their pre-formed opinion here.)
The attack of opportunity has to happen before the move, for the simple reason that most melee weapons have a reach of 5 feet. If the attack happens *after* the move, the target is 10 feet away and you miss automatically because they're out of range. Since that's not the case, AOO happens before the move.
So, what is the trigger for the AOO? A PC is a heroic-level combatant, beyond the skills of mere mortals. You're watching the enemy in melee combat. You see their eyes flicker, they shift their weight, realign their feet, and begin to move away rather than attack. (Have you ever fenced? Footwork is BIG.) THAT'S your trigger for the AOO - the initial signal and the beginning of the move.
Now we have this process: Target makes physical movements indicating the start of their move, which leaves them open to an attack of opportunity. PC seizes that opportunity and makes an attack while they're vulnerable, but before they get out of range. Enemy is already committed to the move and has no choice but to take the attack as they move.
Trigger -> attack -> movement.
Warcaster specifically lets you use a spell in place of a simple melee attack. Booming blade is a purely verbal component (plus the weapon you're already swinging at the enemy.)
So, in Matrix-style bullet-time: you observe the trigger, and take your attack of opportunity by swinging your weapon and shouting the verbal components of booming blade. You connect, dealing your weapon damage, and the enemy is *instantly* sheathed in booming energy, which is then triggered as they complete their move.
If this is the case, then I think we are on the same page.
"Not all those who wander are lost"
Oh my, I had forgotten about that little tidbit in War Caster. My bad. Sorry.
However, in order to use that mechanic, the conditions for taking an OA have to be there.
Even so, BB has a range of 5 ft, so if BB even works and hits, then the target is within that 5 ft range. When the target gets to 10ft away, the secondary damage is inflicted because they willingly moved.
So, I think there's the argument about specifically where in space the attack takes place - when the target is within 5 ft or 10 ft of the caster. Personally, my interpretation would be that it takes place within the 5 ft. range.
If you wouldn't allow bb to trigger (or more likely allow a PC/monster to stop their movements to avoid triggering bb) then it would a paradox about when triggering the OA in the first place.
Target stepped away more than 5' allowing an OA against him & being hit with bb.
Target noticed bb so stopped moving (stayed within 5').
If target stayed within 5' of attacker it wouldnt have triggered the OA in the first place.... so he shouldn't have been attacked at all..