Assume you have the War Caster feat and can cast single-target spells instead of making an attack of opportunity.

The War Caster feat 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.

The description of the green-flame blade spell says:

Make a melee attack with a weapon. On hit, the target suffers the attack's normal effects and fire leaps to a different creature you can see within 5 feet of the original target.

If there were no adjacent target, would you allow a player to use this spell instead of an opportunity attack?

If you said yes and there is an adjacent creature, would the player be allowed to forego the secondary damage to use it?


You can cast green-flame blade in both scenarios, unchanged

The text of War Caster states that the spell

must target only that creature.

Note that this is distinct from

must affect only that creature.

Green-flame blade states that

the target suffers the attack's normal effects and fire leaps to a different creature (distinct from target)

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    \$\begingroup\$ Jeremy Crawford has contradicted this several times in several media. If a spell has any effect on more than one creature, they're all 'targets', even if the spell says it 'targets' one creature and then does stuff to another creature. In one of the Sage Advice bits on the Dragon Talk podcast (the one about 'targeting spells'), he used greenflame blade and the Twinned Spell metamagic (which also requires a spell with only a single target) specifically to talk about this point. \$\endgroup\$ – Darth Pseudonym Jul 12 '18 at 18:54

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