I’ve got a player with Eldritch Knight that just took warcaster and asked me how I would rule this and honestly I’m at a loss.
Scenario:
The 4th level EK hits the target with a BB attack triggering the energy. The enemy doesn’t move immediately and a few turns pass. The enemy eventually decides to move out of range before the EK’s next turn triggering an AoO which the fighter again uses BB. Should the moving enemy take 1d8 only or does the BB stack for 2d8? I know the 5th level boon of BB changes the equation a bit, but I’m only specifically referring to the movement portion.
I’m initially inclined to say it doesn’t stack, but I’m not too sure.
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.
Unless the EK has War Caster, they won't be able to use Booming Blade for the Opportunity Attack when the enemy moves out of reach.
OP said he does have War Caster, hence the question.
Furthermore, if a few turns passed, it would not be able to stack anyway as the effect of Booming Bladewould have expired (emphasis mine):
[...] On a hit, the target suffers the weapon attack’s normal effects and then becomes sheathed in booming energy until the start of your next turn. If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends.
Additional info for bonus points: It's been asked before but if a creature triggers an AoO and somebody uses Booming Blade on said creature and hits, the creature is not forced to continue its movement and trigger BB's secondary effect. You get the AoO before it leaves your reach, once hit, they still have the option to ditch their movement and stay in place, thus not taking the secondary damage since nothing is forcing it to move and it has not yet left your reach when you hit it.
I’ve got a player with Eldritch Knight that just took warcaster and asked me how I would rule this and honestly I’m at a loss.
Scenario:
The 4th level EK hits the target with a BB attack triggering the energy. The enemy doesn’t move immediately and a few turns pass. The enemy eventually decides to move out of range before the EK’s next turn triggering an AoO which the fighter again uses BB. Should the moving enemy take 1d8 only or does the BB stack for 2d8? I know the 5th level boon of BB changes the equation a bit, but I’m only specifically referring to the movement portion.
I’m initially inclined to say it doesn’t stack, but I’m not too sure.
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.
Normally this is the right answer. Effects don't stack. However, in the example provided... that isn't the case. Why?
Because the 1st Booming Blade Triggers immediately when they move. Then the Opp attack goes and adds another application immediately after. Now, if they decide to continue to move after that this new application also triggers, if they only stay at that 5' away mark it doesn't.
Rollback Post to RevisionRollBack
I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
I’ve got a player with Eldritch Knight that just took warcaster and asked me how I would rule this and honestly I’m at a loss.
Scenario:
The 4th level EK hits the target with a BB attack triggering the energy. The enemy doesn’t move immediately and a few turns pass. The enemy eventually decides to move out of range before the EK’s next turn triggering an AoO which the fighter again uses BB. Should the moving enemy take 1d8 only or does the BB stack for 2d8? I know the 5th level boon of BB changes the equation a bit, but I’m only specifically referring to the movement portion.
I’m initially inclined to say it doesn’t stack, but I’m not too sure.
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.
Normally this is the right answer. Effects don't stack. However, in the example provided... that isn't the case. Why?
Because the 1st Booming Blade Triggers immediately when they move. Then the Opp attack goes and adds another application immediately after. Now, if they decide to continue to move after that this new application also triggers, if they only stay at that 5' away mark it doesn't.
The 1st Booming Blade would have expired because they didn't move.
Booming Blade has a duration of one round. "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. On a hit, the target suffers the weapon attack’s normal effects and then becomes sheathed in booming energy until the start of your next turn. If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends."
The spell either ends on your opponents turn "if the target willingly moves 5 feet or more" or at the beginning of your next turn.
It seems there is some confusion in the difference between turns and rounds, the OP says that some turns have passed (i.e. other players or creatures had their turn) and the target voluntarily moves before the beginning of the EK's next turn.
In such a scenario, the first BB movement damage is triggered immediately as the 5ft movement is completed, at this time, an AoO is triggered for the EK who chooses to use BB once more, on a hit they would deal damage and the target would find themselves in the position where they will take the additional BB damage once more should they choose to move another 5ft.
It seems there is some confusion in the difference between turns and rounds, the OP says that some turns have passed (i.e. other players or creatures had their turn) and the target voluntarily moves before the beginning of the EK's next turn.
In such a scenario, the first BB movement damage is triggered immediately as the 5ft movement is completed, at this time, an AoO is triggered for the EK who chooses to use BB once more, on a hit they would deal damage and the target would find themselves in the position where they will take the additional BB damage once more should they choose to move another 5ft.
Opportunity attacks are triggered before the target moves, otherwise the target would be out of range.
It seems there is some confusion in the difference between turns and rounds, the OP says that some turns have passed (i.e. other players or creatures had their turn) and the target voluntarily moves before the beginning of the EK's next turn.
In such a scenario, the first BB movement damage is triggered immediately as the 5ft movement is completed, at this time, an AoO is triggered for the EK who chooses to use BB once more, on a hit they would deal damage and the target would find themselves in the position where they will take the additional BB damage once more should they choose to move another 5ft.
Opportunity attacks are triggered before the target moves, otherwise the target would be out of range.
Opportunity attacks are a paradox. They’re triggered by a creature moving out of your reach, but occur just before the creature leaves your reach.
It seems there is some confusion in the difference between turns and rounds, the OP says that some turns have passed (i.e. other players or creatures had their turn) and the target voluntarily moves before the beginning of the EK's next turn.
In such a scenario, the first BB movement damage is triggered immediately as the 5ft movement is completed, at this time, an AoO is triggered for the EK who chooses to use BB once more, on a hit they would deal damage and the target would find themselves in the position where they will take the additional BB damage once more should they choose to move another 5ft.
Opportunity attacks are triggered before the target moves, otherwise the target would be out of range.
Opportunity attacks are a paradox. They’re triggered by a creature moving out of your reach, but occur just before the creature leaves your reach.
Yeah, assuming grid for simplicity here... you must use the 5' movement to leave the square, which then triggers the opportunity attack, which somehow happens before you leave the square you've already left. Because otherwise you couldn't ever hit with opportunity attacks because they'd be too far away when it triggered. Calling them a paradox is apt.
Rollback Post to RevisionRollBack
I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
Anyway, what that does is makes adjudicating order in time difficult, as now there are a couple of different wordings on timing for when these things occur. If the OA and the booming blade damage were at the same time, the ruling is easy: "the creature whose turn it is determines the order of simultaneous events." That means the creature could decide to take the OA first, and only have one BB rider on them whenever they continue stepping away (since they don't stack).
Since the wordings are slightly different though, it is hard to know if they occur at the same time: is "for all intents and purposes the same time" good enough or does it need to be "identical wording same time"?
Or does that mean that since the OA actually occurs before the creature has moved 5', then the OA cantrip must replace the rider of the previous one?
Anyway, what that does is makes adjudicating order in time difficult, as now there are a couple of different wordings on timing for when these things occur. If the OA and the booming blade damage were at the same time, the ruling is easy: "the creature whose turn it is determines the order of simultaneous events." That means the creature could decide to take the OA first, and only have one BB rider on them whenever they continue stepping away (since they don't stack).
Since the wordings are slightly different though, it is hard to know if they occur at the same time: is "for all intents and purposes the same time" good enough or does it need to be "identical wording same time"?
Or does that mean that since the OA actually occurs before the creature has moved 5', then the OA cantrip must replace the rider of the previous one?
In this scenario, since the OA occurs while the creature is still within 5' reach and has not yet moved more then 5 feet (remember, the rider effect is triggered only after at least 5' of movement), personally, I would rule that they would take the OA first while it is still within 5 feet as it starts its movement and then take only one rider effect after moving more then 5 feet and would not stack them.
Anyway, what that does is makes adjudicating order in time difficult, as now there are a couple of different wordings on timing for when these things occur. If the OA and the booming blade damage were at the same time, the ruling is easy: "the creature whose turn it is determines the order of simultaneous events." That means the creature could decide to take the OA first, and only have one BB rider on them whenever they continue stepping away (since they don't stack).
Since the wordings are slightly different though, it is hard to know if they occur at the same time: is "for all intents and purposes the same time" good enough or does it need to be "identical wording same time"?
Or does that mean that since the OA actually occurs before the creature has moved 5', then the OA cantrip must replace the rider of the previous one?
In this scenario, since the OA occurs while the creature is still within 5' reach and has not yet moved more then 5 feet (remember, the rider effect is triggered only after at least 5' of movement), personally, I would rule that they would take the OA first while it is still within 5 feet as it starts its movement and then take only one rider effect after moving more then 5 feet and would not stack them.
You have to spend your movement to leave your square and enter the next one though. That's step 1 actually: move 5'.
Sequence:
Move 5'.
Trigger Booming Blade.
Movement causes you to leave reach, triggers OA.
Warcaster subs BB instead of OA.
Timetravelling BB hits before they leave, because reach.
Now they're 5' away and have a new BB effect on them.
Rollback Post to RevisionRollBack
I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
I'm probably forgetting some rule, but I think this is easier if we forget the optional rule "playing on a grid".
For medium creatures with normal 5' reach, the closest they can stand together is 2.5' otherwise they will be in each other's space. The creature affected by booming blade only has to move >2.5' to leave the EK's reach triggering the opportunity attack, but not the booming blade extra damage. New booming blade gets applied, but not double booming blade.
Anyway, what that does is makes adjudicating order in time difficult, as now there are a couple of different wordings on timing for when these things occur. If the OA and the booming blade damage were at the same time, the ruling is easy: "the creature whose turn it is determines the order of simultaneous events." That means the creature could decide to take the OA first, and only have one BB rider on them whenever they continue stepping away (since they don't stack).
Since the wordings are slightly different though, it is hard to know if they occur at the same time: is "for all intents and purposes the same time" good enough or does it need to be "identical wording same time"?
Or does that mean that since the OA actually occurs before the creature has moved 5', then the OA cantrip must replace the rider of the previous one?
In this scenario, since the OA occurs while the creature is still within 5' reach and has not yet moved more then 5 feet (remember, the rider effect is triggered only after at least 5' of movement), personally, I would rule that they would take the OA first while it is still within 5 feet as it starts its movement and then take only one rider effect after moving more then 5 feet and would not stack them.
You have to spend your movement to leave your square and enter the next one though. That's step 1 actually: move 5'.
Sequence:
Move 5'.
Trigger Booming Blade.
Movement causes you to leave reach, triggers OA.
Warcaster subs BB instead of OA.
Timetravelling BB hits before they leave, because reach.
Now they're 5' away and have a new BB effect on them.
Your 3rd point is false, taken from the subsection for opportunity attacks in chapter 9 of the PHB:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
This effectively means that they don't trigger the BB rider effect until after the OA, thus only applying it once.
Anyway, what that does is makes adjudicating order in time difficult, as now there are a couple of different wordings on timing for when these things occur. If the OA and the booming blade damage were at the same time, the ruling is easy: "the creature whose turn it is determines the order of simultaneous events." That means the creature could decide to take the OA first, and only have one BB rider on them whenever they continue stepping away (since they don't stack).
Since the wordings are slightly different though, it is hard to know if they occur at the same time: is "for all intents and purposes the same time" good enough or does it need to be "identical wording same time"?
Or does that mean that since the OA actually occurs before the creature has moved 5', then the OA cantrip must replace the rider of the previous one?
In this scenario, since the OA occurs while the creature is still within 5' reach and has not yet moved more then 5 feet (remember, the rider effect is triggered only after at least 5' of movement), personally, I would rule that they would take the OA first while it is still within 5 feet as it starts its movement and then take only one rider effect after moving more then 5 feet and would not stack them.
You have to spend your movement to leave your square and enter the next one though. That's step 1 actually: move 5'.
Sequence:
Move 5'.
Trigger Booming Blade.
Movement causes you to leave reach, triggers OA.
Warcaster subs BB instead of OA.
Timetravelling BB hits before they leave, because reach.
Now they're 5' away and have a new BB effect on them.
Your 3rd point is false, taken from the subsection for opportunity attacks in chapter 9 of the PHB:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
Nope see bolded blue bit.
Rollback Post to RevisionRollBack
I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
Anyway, what that does is makes adjudicating order in time difficult, as now there are a couple of different wordings on timing for when these things occur. If the OA and the booming blade damage were at the same time, the ruling is easy: "the creature whose turn it is determines the order of simultaneous events." That means the creature could decide to take the OA first, and only have one BB rider on them whenever they continue stepping away (since they don't stack).
Since the wordings are slightly different though, it is hard to know if they occur at the same time: is "for all intents and purposes the same time" good enough or does it need to be "identical wording same time"?
Or does that mean that since the OA actually occurs before the creature has moved 5', then the OA cantrip must replace the rider of the previous one?
In this scenario, since the OA occurs while the creature is still within 5' reach and has not yet moved more then 5 feet (remember, the rider effect is triggered only after at least 5' of movement), personally, I would rule that they would take the OA first while it is still within 5 feet as it starts its movement and then take only one rider effect after moving more then 5 feet and would not stack them.
You have to spend your movement to leave your square and enter the next one though. That's step 1 actually: move 5'.
Sequence:
Move 5'.
Trigger Booming Blade.
Movement causes you to leave reach, triggers OA.
Warcaster subs BB instead of OA.
Timetravelling BB hits before they leave, because reach.
Now they're 5' away and have a new BB effect on them.
Your 3rd point is false, taken from the subsection for opportunity attacks in chapter 9 of the PHB:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
Nope see bolded blue bit.
You're going to completely ignore the part at the end that says that the attack occurs before they leave your reach? Honestly, do what you want and abuse the rules as much as you want but the text here is clear that the attack is triggered by the movement yes but the hit connects before leaving reach, before the 5 foot of movement is expanded and before the BB rider effect is triggered.
If you want to homebrew it and let it work, be my guest, but you can't say your interpretation is RAW because it most certainly is not.
Anyway, what that does is makes adjudicating order in time difficult, as now there are a couple of different wordings on timing for when these things occur. If the OA and the booming blade damage were at the same time, the ruling is easy: "the creature whose turn it is determines the order of simultaneous events." That means the creature could decide to take the OA first, and only have one BB rider on them whenever they continue stepping away (since they don't stack).
Since the wordings are slightly different though, it is hard to know if they occur at the same time: is "for all intents and purposes the same time" good enough or does it need to be "identical wording same time"?
Or does that mean that since the OA actually occurs before the creature has moved 5', then the OA cantrip must replace the rider of the previous one?
In this scenario, since the OA occurs while the creature is still within 5' reach and has not yet moved more then 5 feet (remember, the rider effect is triggered only after at least 5' of movement), personally, I would rule that they would take the OA first while it is still within 5 feet as it starts its movement and then take only one rider effect after moving more then 5 feet and would not stack them.
You have to spend your movement to leave your square and enter the next one though. That's step 1 actually: move 5'.
Sequence:
Move 5'.
Trigger Booming Blade.
Movement causes you to leave reach, triggers OA.
Warcaster subs BB instead of OA.
Timetravelling BB hits before they leave, because reach.
Now they're 5' away and have a new BB effect on them.
Your 3rd point is false, taken from the subsection for opportunity attacks in chapter 9 of the PHB:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
Nope see bolded blue bit.
You're going to completely ignore the part at the end that says that the attack occurs before they leave your reach?
I didn't ignore it, see step 5.
Honestly, do what you want and abuse the rules as much as you want but the text here is clear that the attack is triggered by the movement yes but the hit connects before leaving reach, before the 5 foot of movement is expanded and before the BB rider effect is triggered.
Strange sentence. But the red part is entirely incorrect and why you're having trouble here.
If you want to homebrew it and let it work, be my guest, but you can't say your interpretation is RAW because it most certainly is not.
I homebrew a lot of things! This isn't one of them.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
Anyway, what that does is makes adjudicating order in time difficult, as now there are a couple of different wordings on timing for when these things occur. If the OA and the booming blade damage were at the same time, the ruling is easy: "the creature whose turn it is determines the order of simultaneous events." That means the creature could decide to take the OA first, and only have one BB rider on them whenever they continue stepping away (since they don't stack).
Since the wordings are slightly different though, it is hard to know if they occur at the same time: is "for all intents and purposes the same time" good enough or does it need to be "identical wording same time"?
Or does that mean that since the OA actually occurs before the creature has moved 5', then the OA cantrip must replace the rider of the previous one?
In this scenario, since the OA occurs while the creature is still within 5' reach and has not yet moved more then 5 feet (remember, the rider effect is triggered only after at least 5' of movement), personally, I would rule that they would take the OA first while it is still within 5 feet as it starts its movement and then take only one rider effect after moving more then 5 feet and would not stack them.
You have to spend your movement to leave your square and enter the next one though. That's step 1 actually: move 5'.
Sequence:
Move 5'.
Trigger Booming Blade.
Movement causes you to leave reach, triggers OA.
Warcaster subs BB instead of OA.
Timetravelling BB hits before they leave, because reach.
Now they're 5' away and have a new BB effect on them.
Your 3rd point is false, taken from the subsection for opportunity attacks in chapter 9 of the PHB:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
Nope see bolded blue bit.
You're going to completely ignore the part at the end that says that the attack occurs before they leave your reach? Honestly, do what you want and abuse the rules as much as you want but the text here is clear that the attack is triggered by the movement yes but the hit connects before leaving reach, before the 5 foot of movement is expanded and before the BB rider effect is triggered.
If you want to homebrew it and let it work, be my guest, but you can't say your interpretation is RAW because it most certainly is not.
I didn't ignore it, see step 5.
You're still ignoring it by stating that the first BB rider effect triggers before the OA BB hits despite not yet having moved 5 feet when the attack lands. OAs are special reactions in the sense that the reaction is taken after the trigger (moving out of reach) as usual but its effects are applied before the trigger.
This "time traveling" BB as you call it would thus be applied on a target already under the effects of BB and before the rider effect was triggered. It would thus only apply once... not twice.
I’ve got a player with Eldritch Knight that just took warcaster and asked me how I would rule this and honestly I’m at a loss.
Scenario:
The 4th level EK hits the target with a BB attack triggering the energy. The enemy doesn’t move immediately and a few turns pass. The enemy eventually decides to move out of range before the EK’s next turn triggering an AoO which the fighter again uses BB. Should the moving enemy take 1d8 only or does the BB stack for 2d8? I know the 5th level boon of BB changes the equation a bit, but I’m only specifically referring to the movement portion.
I’m initially inclined to say it doesn’t stack, but I’m not too sure.
It won't stack. This is from the PHB.
Thanks. Don’t know how I missed that.
Unless the EK has War Caster, they won't be able to use Booming Blade for the Opportunity Attack when the enemy moves out of reach.
Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)
OP said he does have War Caster, hence the question.
Furthermore, if a few turns passed, it would not be able to stack anyway as the effect of Booming Bladewould have expired (emphasis mine):
[...] On a hit, the target suffers the weapon attack’s normal effects and then becomes sheathed in booming energy until the start of your next turn. If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends.
Additional info for bonus points: It's been asked before but if a creature triggers an AoO and somebody uses Booming Blade on said creature and hits, the creature is not forced to continue its movement and trigger BB's secondary effect. You get the AoO before it leaves your reach, once hit, they still have the option to ditch their movement and stay in place, thus not taking the secondary damage since nothing is forcing it to move and it has not yet left your reach when you hit it.
Normally this is the right answer. Effects don't stack. However, in the example provided... that isn't the case. Why?
Because the 1st Booming Blade Triggers immediately when they move. Then the Opp attack goes and adds another application immediately after. Now, if they decide to continue to move after that this new application also triggers, if they only stay at that 5' away mark it doesn't.
I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
The 1st Booming Blade would have expired because they didn't move.
Booming Blade has a duration of one round. "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. On a hit, the target suffers the weapon attack’s normal effects and then becomes sheathed in booming energy until the start of your next turn. If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends."
The spell either ends on your opponents turn "if the target willingly moves 5 feet or more" or at the beginning of your next turn.
It seems there is some confusion in the difference between turns and rounds, the OP says that some turns have passed (i.e. other players or creatures had their turn) and the target voluntarily moves before the beginning of the EK's next turn.
In such a scenario, the first BB movement damage is triggered immediately as the 5ft movement is completed, at this time, an AoO is triggered for the EK who chooses to use BB once more, on a hit they would deal damage and the target would find themselves in the position where they will take the additional BB damage once more should they choose to move another 5ft.
Opportunity attacks are triggered before the target moves, otherwise the target would be out of range.
Opportunity attacks are a paradox. They’re triggered by a creature moving out of your reach, but occur just before the creature leaves your reach.
Yeah, assuming grid for simplicity here... you must use the 5' movement to leave the square, which then triggers the opportunity attack, which somehow happens before you leave the square you've already left. Because otherwise you couldn't ever hit with opportunity attacks because they'd be too far away when it triggered. Calling them a paradox is apt.
I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
Anyway, what that does is makes adjudicating order in time difficult, as now there are a couple of different wordings on timing for when these things occur. If the OA and the booming blade damage were at the same time, the ruling is easy: "the creature whose turn it is determines the order of simultaneous events." That means the creature could decide to take the OA first, and only have one BB rider on them whenever they continue stepping away (since they don't stack).
Since the wordings are slightly different though, it is hard to know if they occur at the same time: is "for all intents and purposes the same time" good enough or does it need to be "identical wording same time"?
Or does that mean that since the OA actually occurs before the creature has moved 5', then the OA cantrip must replace the rider of the previous one?
In this scenario, since the OA occurs while the creature is still within 5' reach and has not yet moved more then 5 feet (remember, the rider effect is triggered only after at least 5' of movement), personally, I would rule that they would take the OA first while it is still within 5 feet as it starts its movement and then take only one rider effect after moving more then 5 feet and would not stack them.
You have to spend your movement to leave your square and enter the next one though. That's step 1 actually: move 5'.
Sequence:
I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
I'm probably forgetting some rule, but I think this is easier if we forget the optional rule "playing on a grid".
For medium creatures with normal 5' reach, the closest they can stand together is 2.5' otherwise they will be in each other's space. The creature affected by booming blade only has to move >2.5' to leave the EK's reach triggering the opportunity attack, but not the booming blade extra damage. New booming blade gets applied, but not double booming blade.
Your 3rd point is false, taken from the subsection for opportunity attacks in chapter 9 of the PHB:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
This effectively means that they don't trigger the BB rider effect until after the OA, thus only applying it once.
Nope see bolded blue bit.
I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
You're going to completely ignore the part at the end that says that the attack occurs before they leave your reach? Honestly, do what you want and abuse the rules as much as you want but the text here is clear that the attack is triggered by the movement yes but the hit connects before leaving reach, before the 5 foot of movement is expanded and before the BB rider effect is triggered.
If you want to homebrew it and let it work, be my guest, but you can't say your interpretation is RAW because it most certainly is not.
I didn't ignore it, see step 5.
Strange sentence. But the red part is entirely incorrect and why you're having trouble here.
I homebrew a lot of things! This isn't one of them.
I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
You're still ignoring it by stating that the first BB rider effect triggers before the OA BB hits despite not yet having moved 5 feet when the attack lands. OAs are special reactions in the sense that the reaction is taken after the trigger (moving out of reach) as usual but its effects are applied before the trigger.
This "time traveling" BB as you call it would thus be applied on a target already under the effects of BB and before the rider effect was triggered. It would thus only apply once... not twice.