The booming blade and green-flame blade cantrips from the Sword Coast Adventurer's Guide both have the particularity of including a single melee weapon attack as part of their casting.

Several martial abilities, like Extra Attack or the Martial Arts’s bonus attack, do not work with these cantrips because they require the Attack action to be used.

However, the Hunter Ranger's Horde Breaker ability (which lets you do an additional attack on another target with the same weapon once per turn) only requires an attack with a weapon, not necessarily the Attack action. Therefore, I’m wondering if a Hunter Ranger with Horde Breaker could get that additional-attack-on-different-target-with-same-weapon when using booming blade or green-flame blade.

Yes

Horde Breaker states (PHB, p. 93, bold added):

Once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature that is within 5 feet of the original target and within range of your weapon.

For both Green Flame Blade and Booming Blade, you do make a weapon attack.

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. (Sword Coast Adventurer's Guide, p. 142 & 143, bold added)

Note that you won't get any of the benefits of either spell a second time for the second attack.

  • 1
    Might be a good idea to add if you get the green flame blade effect on the horde breaker attack – NautArch Jun 21 at 23:20
  • @NautArch indeed... I was about to comment the same thing (just thought about it). – Gael L Jun 21 at 23:21
  • And of course, if you get the effect of Booming Blade on the additional attack. – Gael L Jun 21 at 23:22
  • I fully agree, but I think an improvement would be to say why the spell doesn't benefit the 2nd attack to close the loop. – NautArch Jun 22 at 14:21

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