Warcaster

Hello,

            I remember at one point there was a great deal of discussion about the warcaster reaction being able to only cast a spell that could target only one creature ( I think). But as written now, 

Reactive Spell. When a creature provokes an Opportunity Attack from you by leaving your reach, you can take a Reaction to cast a spell at the creature rather than making an Opportunity Attack. This spell must have a casting time of one action and must target only that creature. 

So, can I cast a spell that has the potential for targeting multiple creatures, but is focused entirely upon this one ( Magic Missile, Slow, etc)? 

Thanks, 

     SirEvan

That sounds like it, any spell that won’t allow you to target a single individual would probably not be allowed, like Fireball.

The wording says that the spell you cast "must target only that creature". The spell can be capable of targeting multiple creature, but you must aim it only at the creature that provoked the Opportunity Attack. A good example of a spell like this would be Scorching Ray. As long as you direct all of the beams at the creature who provoked the Opportunity Attack, you can cast it with Reactive Spell.

AoE spells like Fireball actually target a point, not a creature, so they are inelligible for casting with the Reactive Spell feature even if they would only hit that one creature (no other targets in the area).

I have a different interpretation regarding Range and Targets.

Fireball has a point of origin (its Range), but the targets are the creatures affected by the spell's Effect. By this reading or ruling, you could technically use War Caster with Fireball as long as you only target one creature.

Range

A spell’s range indicates how far from the spellcaster the spell’s effect can originate, and the spell’s description specifies which part of the effect is limited by the range.

Effects

The effects of a spell are detailed after its duration entry. Those details present exactly what the spell does, which ignores mundane physical laws; any outcomes beyond those effects are under the DM’s purview. Whatever the effects, they typically deal with targets, saving throws, attack rolls, or all three, each of which is detailed below.

Targets

A typical spell requires the caster to pick one or more targets to be affected by the spell’s magic. A spell’s description says whether the spell targets creatures, objects, or something else. [...]

But I know this interpretation of Range and Targets isn't read the same way by everyone, so I'm leaving some threads here with similar or different interpretations:

- 2024 Evoker Wizard, Potent Cantrip and Weapon Cantrips (starting from comment #9)
- Is the attack from True Strike both a Weapon and Spell Attack ? (starting from comment #20)
- True strike and Arcane Grimoire stack?
- True strike and overthinking it (starting from comment #7)
- Use of Booming Blade during a Reaction

The first problem with Fireball is that it will generally target at least two creatures because its radius is larger than your Reach for the Opportunity Attack.

The second problem is that you have to make a decision about whether Fireball is legal before you know whether or not it potentially could be. There very well could be an Invisible enemy within the radius that you don't know about. Since you can't have rules determined by future knowledge, this means we really need to read the rule as "only potentially have one target".

Quote from SirEvangelos >>

So, can I cast a spell that has the potential for targeting multiple creatures, but is focused entirely upon this one ( Magic Missile, Slow, etc)?

There's no explicit RAW answer, from the books or Sage Advice, in 5e or 5.5e.

There are some tweets or such from Crawford from a few years ago suggesting this is fine (he used Green-Flame Blade as an example, where you choose to forego the secondary damage against a second target). So, in principle, there is an argument that you could do this with any spell as long as you can and do only target one thing.

As mentioned, AoE spells that might target a creature or object you don't know is there are a problem.

(People also love to get hung up on the difference between "range" and "target" and such, which makes this more complicated than it needs to be.)

Regarding the Dev tweets, here are the ones I know of that relate to this topic, in case they're useful for you @SirEvangelos or anyone:

@JeremyECrawford A note about D&D spells with a range of "Self (XYZ)": the parenthetical—which says "5-foot radius," "15-foot cone," or something else—means you are the spell's point of origin, but you aren't necessarily its target. You're creating an effect that originates in your space.
@OregonRolledA20  :( still very sad that y'all nerfed booming blade to not work with war caster
@JeremyECrawford Booming blade works with War Caster.
@DMdandanfieldng Related to this note and the new errata to spells like Booming Blade and Green-Flame Blade - does the new range of "Self (5-foot radius)" on these spells mean they no longer work with War Caster?
@JeremyECrawford The Booming Blade spell continues to work with the War Caster feat. The spell targets one creature. The Green-Flame Blade spell continues to work with War Caster if you forgo targeting a second creature with the green fire.

And this is similar to the Scorching Ray example that Sequilonis mentioned, but using Eldritch Blast:

@thomas_conners Can warcaster be used to cast a lvl 5+ Eldritch Blast as long as all the rays target only the provoking creature?
@JeremyECrawford Yes

 
EDIT: for clarity.