Can Green-Flame Blade be cast twice with the Hunter ranger's Horde Breaker ability?

Question

I am currently working on a multiclassed Ranger/Sorcerer character, and I have decided to take the Hunter ranger archetype. The Horde Breaker option for the Hunter's Prey feature seems to allow me to use the green-flame blade spell twice in one round.

Horde Breaker reads:

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.

This ability does not mention taking the Attack action, just making an attack.

The green-flame blade cantrip (SCAG, p. 143) reads:

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 green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier.

Green-flame blade also requires you to make an attack roll.

Am I missing anything, or can I cast green-flame blade twice in one round using this ability?

Answer 1

Horde Breaker can work with green-flame blade, but not in the way you think it does

When you make a weapon attack, Horde Breaker lets you make another attack with the same weapon against a different target 5 feet from the first one, as stated in the quote. It does not let you cast a spell - all it lets you do is make an additional weapon attack.

Green-flame blade, as mentioned in the quote, involves a melee attack with a weapon as part of it, with additional damage leaping to another target 5 feet from the one you hit with the attack.

However, let's say your multiclassed ranger/sorcerer character casts green-flame blade on target A (who has another enemy of yours, target B, next to them and also within reach of your weapon). Then as part of the cantrip, your character will make a weapon attack with a melee weapon against target A. This meets the requirement for Horde Breaker:

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.

Thus, you can make a second attack with the same weapon against target B.

In addition, as a result of the green-flame blade cantrip, if you hit target A with the first attack you make as part of the cantrip, then target B will automatically take damage from the green fire leaping to them.

If both attacks hit, then a 4th-level character (the lowest level at which you could do this, as a multiclassed Hunter ranger 3/sorcerer 1) would do the weapon attack's normal damage to target A, and target B would take the weapon attack's normal damage as well as fire damage equal to your spellcasting ability modifier. (One level later, at character level 5, both targets would take an additional 1d8 fire damage.)

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In short: Horde Breaker lets you make one extra weapon attack against a target next to the first - nothing more. The weapon attack made as part of green-flame blade can meet the requirement to let you make this second weapon attack, but you can only cast the spell once with your action.

Answer 2

You get to make a second attack, but it doesn't benefit from Green-flame Blade

You're right in that when you cast green-flame blade, you must make a weapon attack, and you can use that weapon attack to trigger Horde Breaker and make a second weapon attack against another adjacent target. Horde Breaker's wording doesn't require the use of the Attack action specifically, so it doesn't matter that you made the weapon attack as part of a Cast a Spell action.

However, all this grants you the ability to do is make a second attack with the same physical weapon. The spell, green-flame blade, does not produce some kind of new weapon you could possibly use twice; it simply adds a rider to the attack you make as part of the spell that confers an extra effect on a hit. The second attack you might make with Horde Breaker is not part of the spell and doesn't confer any ability to Cast a Spell a second time - you get one free attack, not one free Action.

Answer 3

No you cannot cast it twice.

The reason is because, as you say, you need to Cast Green-flame blade, but Horde Breaker only allows you to make another attack with the weapon you used.

Which means that Horde Breaker will trigger off of casting Green-flame blade and allows you to swing the same weapon again, but you can only make a regular attack with it, not cast a spell, or use extra attacks, or other abilities.