“Your [wizards] were so preoccupied with whether they could, they didn’t stop to think if they should.” That, right there, could be said about any number of Dungeons & Dragons monstrosities, and it’s certainly true of the dracohydra, which according to Fizban’s Treasury of Dragons is the end result of the question, “Do you think we could make our own Tiamat?”
The dracohydra walks, swims and flies; I’m surprised it doesn’t also burrow and climb as well, but those first three are plenty. Ability-wise, it’s an unambiguous brute, with extraordinary Strength and Constitution and not a whole lot else. Like the hydra, the chassis it’s built on, it’s not tactically complex, and making a combat encounter with one interesting is going to require some additional elements, like environmental hazards, time pressure, distracting vermin—or, as suggested by the flavor text, the chuckleheaded mage who brought it into being.
On any given turn, the dracohydra prefers to use its Prismatic Breath, if that ability is available. It’s on a 4–6 recharge, meaning it will be available approximately every other turn, and the dracohydra should have no trouble positioning itself to hit at least six opponents (or all of them, whichever is less) in the cone. As the action is written, you as the Dungeon Master choose the type of damage it deals to all its targets, although since it unleashes “a single breath of multicolored energy,” I think it might be fun to affect each target it hits with a random type of damage, on the prismatic spray principle. In repositioning to strike the greatest number of targets, the dracohydra doesn’t concern itself with opportunity attacks; in fact, since it can fly and its Bite attack has a 10-foot reach, I’d expect it to spend most of its time in the air. Also, when it comes to opportunity attacks, the dracohydra gives as good as it gets, thanks to its multiple reactions.
Aside from Prismatic Breath, the dracohydra’s only other attack option is Multiattack: one chomp for each head. Thirty damage in a single turn kills a head, but unless someone is dealing radiant damage to it, two heads grow back in its place—the hydra principle again. Each new head comes with 10 regained hit points, so it is possible to kill a dracohydra without cutting off all of its heads, but it’s time-consuming and dangerous: the more heads, the more damage it dishes out.
In terms of positioning, the dracohydra sticks to the air, 10 feet above as many foes as possible, unless it’s forced down to earth. Alternatively, in a battle on water, the dracohydra can lurk just beneath the surface, bringing the rules for underwater combat (Player’s Handbook, chapter 9) into play. This tactic is advantageous to the dracohydra: thanks to its swimming speed, its attacks are unencumbered, but its opponents’ ranged weapon attacks, if aimed into the water, will be at disadvantage unless they’re on the approved weapon list.
This rote procedure is all the dracohydra has the Intelligence for, and its Wisdom isn’t quite high enough that it can resist its truculent urges and steer clear of a group of foes who are too tough for it to take on. It ought to have a normal self-preservation instinct, but given the process and intent behind its creation, I think it’s too bad-tempered ever to flee, no matter how much damage it takes.
Next: draconic shards.