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Hydras are inspired by greek mythology, which as far as I know only had the one. In D&D, I see them on the Rules Cyclopedia, which means they’ve been around at least since the days of BECMI and probably since the beginning. They appear in both books here.
A hydra is a giant reptilian monster, with a snake-like body and a number of dragon-like heads. The first hydras sprang forth from the poisonous blood of Bryakus, one of the most powerful primordials slain during the Dawn War. They spread throughout the world and the planes and have thrived ever since. There are many species of hydras, each with a different set of abilities and a different number of heads. No species has less than four, though.
Hydras are nonsapient, but they’re still extremely dangerous predators that require a lot of food to survive. Most species are good swimmers, and though they can’t breathe underwater only one of the heads needs to surface to avoid drowning. Hydras can survive on pretty much any environment as long as they have access to food.
Despite their innate ferocity, hydras actually take quite well to training if the trainer manages to survive the turbulent early stages of the process. Giants, minotaurs and powerful spellcasters are the most frequent hydra owners, though sometimes a hydra will “adopt” someone more unexpected than that, like a goblin leader.
The mythological Hydra was notorious for its ability to regrow its severed heads, and here we have an interesting lore divergence between the two books: the Monster Manual describes this as a persistent rumor that has so far proved untrue; and the Monster Vault says it’s true after all.
Both books agree hydras are threats for the paragon tier and above. They also give all hydras trained Perception, All-Around Vision, and Darkvision. Hydras always have a swim speed unless it goes against the theme of their specific abilities, though they’re not Aquatic.
All hydras in both books have Threatening Reach, allowing them to make opportunity attacks out to their full reach instead of just against adjacent enemies. They also have the Many-Headed trait, which does different things in each book. In the MM, it means that a stun or daze only causes the hydra to lose one of its attacks on its next turn (though this stacks). In the MV, it means the hydra can take free actions even while dazed or stunned, which is powerful because “bite everyone in reach” is a free action for them.
The lore divergence above translates into a numbers divergence as well. MM hydras have no head-severing mechanics at all, and more hydras powerful hydras have more heads. MV hydras always start with four heads, and lose one each at 75%, 50%, and 25% HP. At the start of the next turn after losing a head, two others grow in its place. This increases their number of attacks, but skilled PCs might prevent it from happening by means described in the hydra stat block.
MM hydras also tend to have considerably more HP for their level, since they use the old math. That’s another thing you should fix if you want to add them to your game.
The basic model is a Large Natural Beast with the Reptile and Water keywords. It’s also a Level 10 Solo Brute with 432 HP and the common traits described above. Its land speed is 5, and it swim speed 10.
The hydra has four heads to start with, and it lose one at 324, 216, and 108 HP. Unless it takes fire or acid damage after losing a head, it will grow two new ones on its next turn.
The hydra fights by biting, and its Reach 2 bites are kinda weak to make up for the fact that there are so many of them. The Hydra Fury at-will attack allows it to bite once per head it currently has, gaining a damage bonus if it only has 2 or 1 left. As an at-will free action, the hydra can use two bites against anyone who ends their movement within reach of it.
Fen Hydra (MM)
This is the basic hydra from the MM. It’s a Large Natural Beast (reptile) with all common traits for MM hydras. A Level 12 Solo Brute with 620 HP, it’s otherwise quite similar to the MV hydra, except it lacks a head-severing mechanic. It has four heads throughout the whole fight, so its Hydra Fury always allows it to make 4 attacks.
Flamekiss Hydra (MV)
A fire-themed hydra, this is a Large Natural Beast (reptile) and a Level 12 Solo Brute with 496 HP. Flamekiss hydras have a land speed of 5 and no swim speed, but also have Resist 10 Fire. They lose heads at 372, 248 and 124 HP, and PCs can prevent them from growing more by dealing cold or acid damage before their next turn starts.
Flamekiss hydras have the same attacks as the base model, plus Flame Kiss (Close Blast 3 vs. Reflex) which deals immediate and ongoing fire damage (save ends). This recharges every time the hydra loses a head. It can be used in place of a bite during a Hydra Fury combo, and benefits from the damage bonus if only 1 or 2 heads remain.
Venom-Maw Hydra (MV)
This is the hydra that most closely resembles the mythological one, which was all about the venom. It’s a Huge Natural Beast (reptile, water) and a Level 17 Solo Brute with 672 HP and all common hydra traits. Its land speed is 7, and its swim speed remains 10.
The venom-maw hydra’s beheading thresholds are at 504, 336 and 168 HP, and it can be prevented from regrowing heads by taking acid or fire damage before the start of its next turn. Its Reach 3 bites deal 10 ongoing poison damage, or 20 if the hydra is bloodied (its blood is also poison).
This hydra can attack at range by spitting venom (Ranged 10 vs. Reflex), which deals poison damage and knocks the target prone on a hit from sheer pressure. Despite being a ranged attack, it does not provoke opportunity attacks. Either the bite or the venomous spit can be used as attacks during a Hydra Fury combo, with the usual damage bonus if only 1 or 2 heads remain.
The venom-maw hydra’s Snapping Jaws only have reach 2, which I think might be a mistake. They should be reach 3 like its normal bites.
Mordant Hydra (MM)
This Huge Natural Beast (reptile) is acid-themed. It’s a Level 18 Solo Brute with 880 HP, Resist 15 Acid, and all common hydra traits. It has a land speed of 6 and a swim speed of 12! It also has six heads.
Mordant hydras fight with Reach 3 bites and Range 10 acid spit (vs. Reflex), and its Hydra Fury allows it to make six of those attacks in any combination. All of its other tricks come from the standard MM hydra abilities.
Primordial Hydra (MM)
This one is likely one of the original hydras born from the blood of Bryakus. It’s a Gargantuan Elemental Beast (reptile), and a Level 25 Solo Brute with 1200 HP. It has eight heads, a land speed of 8 and a swim speed of 16, which makes it faster than just about any other monster in the book when in water. It has Resist 20 to Acid and Fire, and all other common hydra abilities.
The primordial hydra’s bite has Reach 4, and enjoys attack and damage bonuses when used for opportunity attacks. It attacks at range with Flaming Acid Spit (Range 10 vs. Reflex), which obviously does fire and acid damage. Its Hydra Fury gives it eight attacks which can be any combination of those two maneuvers.
All of its other tricks come from the standard abilities.
Sample Encounters and Final Impressions
We have three sample encounters in the MM:
- Level 14, a fen hydra and 3 bog hags. A covey and their pet.
- Level 19, a mordant hydra and a gibbering abomination. No idea about this one.
- Level 26, 1 primordial hydra and 2 earthwind ravager elementals. A scenic wildlife encounter for the deep Elemental Chaos.
Hydras fulfill some of the same dramatic roles as dragons, since they’re giant monsters meant to provide a memorable fight all by themselves. They’re a bit simpler to run. They’re also nonsapient, which is a good thing because it makes their explaining their presence in a location fairly uncomplicated. Dragons are sapient and have agendas; hydras are just hungry.
I love the hydra head management mechanics they came up with for the MV, since they’re a nice callback to the mythological Hydra and what Hercules had to do to kill it. They also provide some subtle ways to tweak the challenge of a fight since you could start a given hydra with more than four heads.
The simpler MM varieties still have their place, too, since you might not want every hydra to behave like the mythological one.