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The War Caster feat allows a single-target spell ("must target only that creature") to be cast instead of a melee strike for an opportunity attack.

Green-Flame Blade appears to have a single target, and then some damage "leaps" onto a creature 5 feet away. The other creature is never referred to as the target, or even "a target".

I'm pretty sure this is solid from the RAW perspective. Is this legit from a RAI perspective?

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    \$\begingroup\$ Related: rpg.stackexchange.com/q/49170/15469 \$\endgroup\$ – Miniman Mar 26 '16 at 12:06
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    \$\begingroup\$ Ultimately I decided against it for the NPC I was writing. My players open enough Pandora's boxes of game breaking mechanics without me adding to the fire. \$\endgroup\$ – Derrek Bertrand Mar 28 '18 at 16:01
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The RAI on this one is a little uncertain from what I can tell.

Jeremy Crawford responded here to a question about using Green-Flame Blade for AoO.

Without a feature like the War Caster feat, a spell can never be cast in place of an opportunity attack.

He doesn't make it incredibly clear, but the implication here is that with the feat this would be possible. However, this is contradicted by a later tweet here, stating that:

Twinned Spell doesn't work with a spell that targets more than 1 creature. Green-flame blade targets 2.

This explicitly states that the spell targets multiple creatures, and therefore doesn't qualify for the War Caster benefit, but again in a different context. My instinct here is to agree with explicit disqualification, rather than implied exception, so I would say the RAI is that you can't use Green-Flame Blade with an opportunity attack.

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    \$\begingroup\$ Wow, JC's comment says Green-flame blade targets 2 creatures, although the spell description doesn't name the second creature as a target, and there's no attack roll or save. \$\endgroup\$ – Tim Grant Mar 27 '16 at 13:31
  • \$\begingroup\$ This is why it confused me. There are clearly two targets, but one is just circumstantial and it is never treated like most spell "targets". \$\endgroup\$ – Derrek Bertrand Mar 28 '16 at 1:36
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    \$\begingroup\$ Twin spell has an added sentence that makes all the difference in those tweets. "To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level." War Caster has no such sentence. \$\endgroup\$ – Voromir Kadien Mar 2 '18 at 16:17
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Yes you can provided you don't hit any other creature with the secondary effect

War Caster Says :

  • 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. The spell must have a casting time of 1 action and must target only that creature.

As long as the only creature getting hit by the spell is the one that provoked the Opportunity Attack it is legal.

In a very similar question Jeremy E. Crawford tweeted this.

The 3rd benefit of War Caster requires you to target only the moving creature. Eldritch blast works as long as all the beams target it.

All Emphasis Mine

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    \$\begingroup\$ A very good point! If Eldritch Blast works, then so too should Green-Flame Blade as long as there is no secondary effect. This does make things a little more context dependent, unfortunately. \$\endgroup\$ – Derrek Bertrand Apr 11 '18 at 21:51
  • \$\begingroup\$ @DerrekBertrand at least you get some extra damage on your OA a better spell might be Booming Blade though. \$\endgroup\$ – Voromir Kadien Apr 14 '18 at 1:31
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This is probably a DM decision. I would say yes as the spell only targets one creature "you must make a melee attack with a weapon against one creature within the spell's range". The second target isn't directly targeted by the spell, but is hit as an additional property of the spell.

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