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The War Caster feat gives the following property:

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.

A Hydra has the following trait:

Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Let's say that, for some reason, someone provokes an opportunity attack from a War Caster Hydra (possible by, say, a War Caster Wizard casting Shapechange to become a Hydra)... what happens?

  1. The War Caster Hydra can either use all its heads for opportunity attacks, or only a single head for an eligible spell

  2. The War Caster Hydra can use all of its heads for opportunity attacks and/or eligible spells

I suspect that the right answer is 1), but it's better to verify.

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  • 1
    \$\begingroup\$ Related: rpg.stackexchange.com/questions/141591/… \$\endgroup\$ – DucksGoMooful Jun 20 at 16:49
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    \$\begingroup\$ sageadvice.eu/2017/06/24/… seems at least pertinent. (Specifically, Crawford seems to say that the hydra gets multiple reactions, but only one opportunity attack per provoke.) \$\endgroup\$ – Ben Barden Jun 20 at 16:56
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    \$\begingroup\$ @ben but the rule for Shapechange says "You retain the benefit of any features from your class" which would include Spellcasting, right? The only limit would be that the Hydra form would have to be able to perform VSM components (which it can only perform SM since it can't speak). \$\endgroup\$ – Rykara Jun 20 at 17:08
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    \$\begingroup\$ @Rykara you're totally right. I was reading the wrong shapechange. You retain all benefits of your class, which specifically includes feats (and, I suppose, level-based stat boosts). \$\endgroup\$ – Ben Barden Jun 20 at 17:10
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DM's call, with a bias towards "You only get one attack/spellcast per provoke."

Setting aside the issue of how you get a hydra who can cast spells as an opportunity attack, it appears that the hydra can only opportunity attack with one head at a time. I know Crawford is no longer a strong rules authority, but his feedback on the matter at least seems like it would be pertinent.

In this twitter post, in response to the following question:

@JeremyECrawford Mr. Crawford, I'm confused about Reactive Heads. If a player provokes an OA from a Hydra, can it atk them with every head?

he gave this answer:

A hydra can have more reactions than normal, but it doesn't change how an opportunity attack works: one attack per opportunity attack. #dnD

While there has been some confusion on interpretation of this tweet, it seems pretty clear to this poster, at least, that if he'd meant "the hydra can totally attack with every one of its heads in response to a single provocation", it would have been worded differently. Additionally, this brings it in line with the hydra behaviors from earlier editions, where the hydra was able to make a great many opportunity attacks, but only once per provoking event.

By this interpretation, then (which is reasonably well-supported from a reading of the text), you would only get one attack or spellcast regardless.

Now, you may reject Mr Crawford's analysis of things. His tweets are not an official source, after all (or at least, not any more). At that point, the needle swings back to "it's not clear". 5e has by design included a number of places where the rules are not entirely clear, with the intent and expectation that when such things arise, they are DM's call.

So, since War Caster is replacing opportunity attacks, and you (probably) only get one of those at a time, the best available answer (outside of DM adjudication) is: the War Caster Hydra can use one head, to either make a single attack or cast a single spell

Of course, that still leaves you with the challenge of finding an attack spell that the Hydra can actually cast, given the issue with Hydra being incapable of performing Verbal components. Perhaps you have three levels of sorcerer with the right sort of metamagic?

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    \$\begingroup\$ I think I can guess where you are going with this, but it doesn't seem you ever get around to explicitly answering the question as asked (about the War Caster feat's ability). Am I missing something? If not, I think connecting the dots here completely will make for a much better answer. \$\endgroup\$ – Rubiksmoose Jun 20 at 17:28
  • \$\begingroup\$ @Rubiksmoose I've tried to make the conclusion clearer. Better? \$\endgroup\$ – Ben Barden Jun 20 at 17:33
  • \$\begingroup\$ Thanks! I think it might help to go a bit further in depth about linking your arguments about OAs to War Caster, but that could just be me. Either way it is a good improvement. \$\endgroup\$ – Rubiksmoose Jun 20 at 17:35

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