Can you combo attacks with the Booming Blade cantrip?

Question

I understand that so long as conditions are met, you can Sneak Attack as part of the melee attack from the Booming Blade cantrip.

Can you dual wield and attack with both weapons after casting Booming Blade? I understand that you probably need something like the War Caster feat and Subtle Spell to pull this off, but I don't know if the melee attack as part of Booming Blade is a bonus action or not.

Can you extend the combo if you had Extra Attack like those from either fighter, ranger, monk, or bard, and use damage-increasing spells like Hex or Hunter's Mark? It's safe to say this build is around Lv 5 or higher.

So is the following combo possible?

• Build (Human Rogue 4, Ranger 3, Sorcerer 3)
• 1st Round: Full round act: cast Hunter's Mark and Hex.
• 2nd Round: cast Booming Blade swing with main hand weapon [weapon + d8 + 2d6 sneak attack]
  then swing again (if with extra attack) [weapon + hunters mark 1d6 + colossus slayer 1d8],
  then swing with off hand [weapon + hunters mark 1d6 + colossus slayer 1d8]

Now if you dip in fighter, can you add Action Surge so you can get two more attacks from your main hand for a possible [2x weapon + hunter's mark 2d6 + colossus slayer 2d8] damage? And 2d8 thunder damage if the targeted enemy goes after the attacker (me)?

Answer 1

No, You cannot combine attacks with booming blade.

Both Booming Blade and Green Flame Blade say the following:

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range

This means that these cantrips do not require you to take the Attack action, they require you to take the Cast a Spell action.

If we look at extra attack, it is defined as:

Beginning at 5th level, you can Attack twice, instead of once, whenever you take the Attack action on Your Turn.

No Attack action is taken, so no extra attacks can be used. Additionally, two weapon fighting states:

When you take the Attack action and attack with a light melee weapon that you’re holding in one hand, you can use a bonus action to attack with a different light melee weapon that you’re holding in the other hand.

Again, no attack action is taken, so you cant use a bonus action to make an off hand attack.

In your scenario, you can cast hunter's mark or hex, and then the hit from the booming blade will trigger those effects, as well as sneak attack (if the conditions are met), but you will be unable to make more attacks after that.

Action surge gives you another whole action, which you can use to cast a spell, like Booming Blade which will give you one more attack, or if you just use a regular attack action, extra attack and two weapon fighting could apply.

Answer 2

That combo will not work, for a number of reasons

First, you cannot cast both Hex and Hunter's Mark. Both require concentration, and both require a Bonus Action to cast, of which you only get one per turn.

Second, and most importantly to the root of your question, casting Booming Blade does not grant any additional attacks, through either the Extra Attack feature, nor Two-weapon Fighting. Both Two-weapon Fighting and Extra attack contain the phrase:

when[ever] you take the Attack action

indicating that they require you to take an entirely different action than the Cast a Spell action that Booming Blade requires.

Third, Colossus Slayer effects only one hit per turn, so you can only add that extra damage once.

Eldritch Knights can almost do this

With seven levels of Fighter (Eldritch Knight), however, you could get close. The Eldritch Knight's seventh level feature grants a Bonus Action attack after casting a cantrip, and the Fighter itself gets the Action Surge feature, which combined would allow to to cast Booming Blade, followed by three (or more, depending on how many levels of Eldritch Knight you take) additional melee weapon attacks, once per short rest.

Answer 3

No, you cannot make an off-hand attack with Booming Blade. Two weapon fighting lets you make an attack with your bonus action when you take the Attack action. Booming Blade is a spell, which requires you to have taken the Cast a Spell action, not the Attack action. It's confusing, because as part of casting the spell you make an attack, but you never made the Attack action, so you never got the chance to use your bonus action to make an off-hand attack.

Answer 4

No, you can't use Extra Attack with booming blade

While I think that Adam's answer is excellent and correct, the Sage Advice Compendium also addresses this question and confirms that the cantrip doesn't work with Extra Attack:

Can you use green-flame blade and booming blade with Extra Attack, opportunity attacks, Sneak Attack, and other weapon attack options?

[...]

Second, neither green-flame blade nor booming blade works with Extra Attack or any other feature that requires the Attack action. Like other spells, these cantrips require the Cast a Spell action, not the Attack action, and they can’t be used to make an opportunity attack, unless a special feature allows you to do so.

Answer 5

Yes, if you're a Bladesinger Wizard.

Generally, you're not able to combine Booming Blade with abilities that trigger off of the Attack Action like Two-Weapon Fighting, because there is a difference between making an attack and taking an Attack Action, and casting Booming Blade generally requires the Cast A Spell Action.

However, as of the release of Tasha's Cauldron of Everything, the Bladesinger Wizard's 6th level ability Extra Attack states the following:

Extra Attack

Starting at 6th level, you can attack twice, instead of once, whenever you take the Attack action on your turn. Moreover, you can cast one of your cantrips in place of one of those attacks.

This would allow them to take the Attack Action and replace one of their attacks with a casting of Booming Blade, which would, in turn, allow them to make an extra attack as a Bonus Action using Two Weapon Fighting. You don't need to worry about the Somatic Components of Booming Blade while doing so, even if you've got a weapon in both hands, because Booming Blade also has a Material component of a weapon worth at least 1sp, so the Somatic Component can be performed using the hand holding the weapon.

Some of the other answers have addressed your misunderstandings of certain class features of other classes, but I just wanted to make this explicitly clear about how this works.

Answer 6

You could use Quickened Spell on booming blade and also cast booming blade as an action to get a total of 2 booming blade attacks, although if you hit the same target twice, only one instance of additional damage would be triggered if the target moved.

You don't need Subtle Spell, because you are holding the material component (the weapon) and can use the same hand that is holding a material component for the somatic component:

A spellcaster must have a hand free to access a spell’s material components—or to hold a spellcasting focus—but it can be the same hand that he or she uses to perform somatic components.

Answer 7

Round One

1st Round: Full round act: cast Hunter's Mark and Hex.

Hunter's mark and hex both take bonus action to cast, so can't be cast on the same turn because you only get one bonus action, and bonus action spells can't be cast as an action. Moreover, both are concentration so can't be kept up at the same time; when you cast the second one, you lose concentration on the first. So, you can get one or the other, not both. And hex is only on the Warlock's spell list anyway, isn't it?

Round Two

2nd Round: cast Booming Blade swing with main hand weapon [weapon + d8 + 2d6 sneak attack] then swing again (if with extra attack) [weapon + hunters mark 1d6 + colossus slayer 1d8], then swing with off hand [weapon + hunters mark 1d6 + colossus slayer 1d8]

Booming blade uses the Cast a Spell action, but the two-weapon fighting rule requires that you took the Attack action:

When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand. You don't add your ability modifier to the damage of the bonus attack, unless that modifier is negative.

Getting Close

The only way to make other attacks with booming blade is to be a Wizard with the Bladesinging subclass. It would hurt HP-wise, but it would allow you to cast booming blade as part of an Attack action because of the Bladesinger's Extra Attack feature, which reads:

You can attack twice, instead of once, whenever you take the Attack action on your turn. Moreover, you can cast one of your cantrips in place of one of those attacks.

That would let you take the Attack action to cast booming blade and make a normal attack that benefits from Sneak Attack, then make an attack with a one-handed weapon in your other hand using your bonus action (using the Two-Weapon Fighting rule).

Answer 8

At 7th level an Eldritch Knight is granted a bonus melee attack following a cantrip based on his/her War Magic feature. If the character casts Booming Blade the cantrip is dependent on a successful melee attack as a part of the spell; here the War Magic feature kicks in and the character may make a second melee attack as a bonus action if he/she has not already taken a bonus action during this same turn.

The above debate comes down to RAW vs. RAI. As the material is written, I believe Reign's interpretation is logical and correct. However, this would seemingly OP characters using these hybrid spell/melee cantrips and stacking Extra Attack and/or a dual wield bonus attack. This seems to break balanced mechanics and is not in keeping with most interpretations of RAI. It would be helpful if WotC would officially rule on this for clarification rather than deferring the matter to individual DM's.

Answer 9

As stated in the D&D Player's Handbook, page 194 & 195:

MAKING AN ATTACK
Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.

1. Choose a target. Pick a target within your attack's range: a creature, an object, or a location.

2. Determine modifiers. The DM determines whether the target has cover and whether you have advantage ar disadvantage against the target. In addition, spel1s, special abilities. and other effects can apply penalties or bonuses to your attack roll.

3. Resolve the attack. you make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.

If there's ever any question whether something you're doing counts as an attack, the rule is simple: if you're making an attack roll, you're making an attack.

The last line makes this answer rather easy: if the spell makes you roll an attack roll, it counts as an attack and you CAN combo attacks after it.

Answer 10

You can use BB or GFB to trigger Extra Attack. In the Player's Handbook, on page 192 it says the following under "Actions in Combat" > "Attack:

The most common action to take in combat is the Attack action, whether you are swinging a sword, firing an arrow from a bow, or brawling with your fists. With this action, you make one melee or ranged attack. See the “Making an Attack” section for the rules that govern attacks. Certain features, such as the Extra Attack feature of the fighter, allow you to make more than one attack with this action.

This indicates the rules governing the Attack action are under the "Making an Attack" section on page 193/194:

If there's ever any question whether something you're doing counts as an attack, the rule is simple: if you're making an attack roll, you're making an attack.

is in the "Making an Attack" section on page 194.

Now looking at booming blade:

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails. On a hit, the target suffers the attack’s normal effects, and it becomes sheathed in booming energy until the start of your next turn. If the target willingly moves before then, it immediately takes 1d8 thunder damage, and the spell ends.

This clearly indicates that you must make an attack roll to use this spell. Under the PHB rules as written on pages 192, 193, and 194. Any time you make an attack roll you are "making an attack". Also, spells like BB or GFB are pretty... unique, because they meet both the Cast a Spell and make an attack criteria. Either way though, you are making an attack.