Question: The War Caster feat (PHB, p. 170) allows a single-target spell ("must target only that creature") to be cast instead of an opportunity attack.

The *Green-Flame Blade* cantrip (SCAG, p. 142) 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".

**Can the *Green-Flame Blade* cantrip be cast as a reaction using the War Caster feat?**

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

Answer: 
Yes, you can, provided you don't hit any other creature with the secondary effect
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The third benefit of the [War Caster](https://www.dndbeyond.com/feats/war-caster) feat says (PHB, p. 170; emphasis mine):

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> * 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.**
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As long as the only creature getting hit by the spell is the one that provoked the opportunity attack, it is legal.

In response to a very similar question on Twitter about using *eldritch blast* (as a character of 5th level or higher) with War Caster, rules designer Jeremy Crawford [tweeted the following](https://twitter.com/jeremyecrawford/status/908469121502736384?lang=en):

> 
> 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.
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This is probably a DM decision. I would say yes as the spell only targets one creature:

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> you must make a melee attack with a weapon against one creature within the spell's range
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The second target isn't directly targeted by the spell, but is hit as an additional property of the spell.