Would every head of a Hydra be affected by the Dragon's Breath spell?

Question

I'm a DM setting up a oneshot. I have an idea to spice up combat a bit by giving a hydra a breath weapon. So far, the least homebrew option I've thought of is to have an enemy spellcaster cast dragon's breath on it. However, a Hydra is stated to start out with 5 heads; does this mean every head could use the breath weapon as one action?

I can only think of two possible solutions that give different answers, but both seem equally viable.

1. The spell specifically says "spew magical energy from its mouth, provided it has one". Since the hydra has five mouths, it can spew from every mouth.
2. The Hydra stat block says "Multiattack: The hydra makes as many bite attacks as it has heads." Since dragon's breath is not a bite attack (and according to Jeremy Crawford, not even an "attack"), it would only be able to use the breath weapon once, despite its number of heads.

The best amalgamation I can think of would be that any head can use its breath weapon, but the creature can only do it once and then can't spend its action using its multiattack. However, this sounds ridiculously under-powered, as it essentially means 4 heads miss a turn. Would allowing every head to use dragon's breath as a result of this spell be too overpowered? Are the attack actions on the hydra's stat block irrelevant in this case, because dragon's breath is not an attack?

In short: How would casting the Dragon's Breath spell on a Hydra work? Also, could each different head have a different damage type?

Answer 1

The entire Hydra is affected, but it can activate the spell only once per turn and can only make one cone at a time

A Hydra (or any monster) has only one action per turn and nothing in its statblock somehow grants it multiple actions. Creating a cone using dragon's breath is its own action as the spell requires a creature to "use an action to exhale energy". Thus the Hydra can only activate dragon's breath once on its turn. The multitude of heads will not help the Hydra here; having them all attack on the same turn requires the Hydra to take the specific action "Multiattack", which it is not doing here.

Only one damaging cone can be made at a time

The dragon's breath spell states:

You touch one willing creature and imbue it with the power to spew magical energy from its mouth, provided it has one. Choose acid, cold, fire, lightning, or poison. Until the spell ends, the creature can use an action to exhale energy of the chosen type in a 15-foot cone. Each creature in that area must make a Dexterity saving throw, taking 3d6 damage of the chosen type on a failed save, or half as much damage on a successful one.

Only creating one cone is likely intended as the singular is used throughout the spell's description: "its mouth", "a cone", "that area".

Outside of possible intent the spell says that the creature creates "a (singular) cone" not "one cone for each head the creature has" and so it creates one, and only one cone even if we read this rule strictly as written.

And so, as established above, the Hydra can use its action to activate this spell only once on each of its turns and that activation creates only one cone.

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To answer your other question ("Could each different head have a different damage type?") , when the spell is cast, the caster choose a single damage type and the creature exhales energy "of the chosen type" (and which deals damage "of the chosen type"). Thus the creature uses whatever damage type the caster chose, and as the heads are part of the creature, each head uses the same damage type as well.

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It is probably the case that any of the heads can make this damaging cone. The spell affects the entire creature and allows it to spew energy from its mouth, it doesn't specify that the same mouth must do this each time (though I doubt the developers thought of this situation). Also, if you heard the sentence "A Hydra can spew energy from its mouth" you would assume this means any and all of its mouths.

If you instead interpreted "its mouth" as strictly singular you run into a problem because a Hydra doesn't have anything that can be described as "its mouth", but only "one of its mouths".

The spell clearly assumes an affected creature has only one mouth and as a result, which mouth(s) of a multi-mouthed creature can activate the spell should be left up to your GM.