When captured by a monster trainer, hydras grant the trainer access to new spells.

The legendary Hydra is a beast of vicious hunger and amazing regenerative powers, renowned for its ability to sprout two new heads when one is decapitated. Resembling a collection of snapping serpents atop a thick, coiling lower body that can run up to 20 feet long, the Hydra is an imposing and brutish menace, lairing in the clammy backwaters of the world and devouring any creature smaller than itself. Its scales are as varied in hue as those of any species of snake, from glossy to dull and from greenish black to crimson, often marked with patterns of stripes, diamonds, and patchworks of color. Some Hydras have heads that are more fanged and viperlike, with smooth and supple scales, while others have rugged hides that are leathery in texture, and elongated crocodilian or even draconic visages. Regardless, most Hydras possess brightly colored frills or crests, traits they take great pride in and use to intimidate prey as well as to scare off potential predators.

Experienced marshwalkers and fisherfolk living in backwoods bayous tell many stories of the Hydra, most of which are cautionary tales of lost friends or distant relations who ran afoul of the creature while in search of a secret fishing hole. According to these stories, the few survivors of such encounters typically abandoned the water and took up farming in the dry hill country, far, far away. While the tales are often told with a wink of humor, a deadly serious moral always underlies them: The Hydra is not a creature to be trifled with. The only thing worse than meeting one by chance is attempting to turn the tables and actively hunt the Hydra. While too dim to understand the human thirst for revenge, a Hydra is all too willing to welcome a foolhardy hunter into its coils, adding its prey's remains to its own stock of grisly trophies hung from the branches around its swampy home. It is only the blessing of the gods that such a terror is not made worse with a sharper mind.

Ecology

Hydras reproduce by asexual budding, with neck rootlets constantly sprouting inside the submuscular fossae at the base of their necks. Each larval rootlet contains yet more endodermal sprouts, and if not released by decapitation, these sprouts begin to grow and mature within the hydra's neck as though it were a serpentine womb. Sometimes these larvae are dislodged into the hydra's throat and hacked up in slimy, leathery cysts, which the Hydra collects and places into swampy nests as if they were eggs until they burst open and release their maturing brood. If neck rootlets instead hatch and mature within the neck, however, the hatchlings actually tear the parent's neck asunder, bursting out of the flaccid tissue like elongated tadpoles. The largest tadpole grafts itself onto the hydra's body and becomes a new head, while the rest of the serpent-spawn are ejected into the surrounding swamp to fend for themselves. Interestingly, larval Hydras are not cannibalistic, and instead teem in dangerous swarms for mutual protection. They continue to bud and grow as they consume prey until the largest mature enough to set out on their own, leaving the weaker remnants vulnerable to predation.

Hydras favor temperate or warm climes and are primarily marsh-dwellers; however, rumors abound of rare Hydras that live among coastal reefs and plague the seas, and the dreaded pyrohydras and cryohydras are known to take up even stranger environs as their homes. While primarily carnivorous, Hydras are not above eating carrion or even sluicing snails, worms, frogs, and the like out of the muck when larger prey is scarce. Because of their highly efficient mode of reproduction, Hydras require huge amounts of food in order to sustain themselves and their bodily functions. Displaying eating habits similar to those of snakes, Hydras typically prefer to devour huge meals all at once and digest them for long periods of time; the average Hydra can consume as much as half its weight in a single day, subsequently fasting for nearly a week before needing to eat again.

Thanks to their size and the juicy tenderness of their fat and flesh, humans and other bipedal creatures-especially stocky ones such as dwarves-are a highly sought after meal for Hydras, which will go to great lengths to devour such beings if given the opportunity. Though they can survive off vermin, vegetation, and plant creatures, Hydras always prefer red meat if given the choice. In times of desperation, Hydras may resort to cannibalism, battling one another in exhausting brawls that only escalate as the rivals sever each other's heads, the victor consuming every scrap of its competitor's remains after the battle is won. Hydras are immune to one another's acidic bile, and those with fiery breath such as pyrohydras make formidable foes against others of their kind, cauterizing necks en masse after numerous severing blows.

Habitat & Society

Hydras are by nature solitary creatures; whatever urges they have for interaction are sufficiently addressed by the constant slither and hiss of their multiple heads. Lacking any need or desire to mate, they have little reason to seek out the kinship of their own species; if anything, Hydras typically drive out others of their own kind that would compete with them for food, though if resources are plentiful, they may tolerate the presence of another Hydra nearby, if only so the two can take down bigger prey and defend themselves from more powerful predators.

A Hydra often lives in symbiosis with small vermin, which crawl over the creature's scales and scrape off and consume algae growing on its skin. These vermin also eradicate even smaller vermin that burrow into the hydra's flesh and feast on its ever-replenishing subcutaneous cells; while the hydra's regenerative powers ensure that the host takes no lasting harm from such parasites, they do irritate the Hydra. It thus welcomes the larger verminous scavengers, sparing them its hungry attentions as they scratch its constant itch. Virtually anything else that moves is fair game for the hydra's monstrous appetite.

The dim intellect and eternal hunger of Hydras make them friends to none. Though they possess an aggressive demeanor by nature, Hydras can be tamed by a hunter of sufficient courage and skill, and the application of magic or wild empathy is a great aid in this endeavor, as is the use of fire or acid in particularly brutal training regimens. Giants are known to train pyrohydras and cryohydras as guardian beasts, though such Hydras are not so much trained as they are simply captured and confined in a space where their anger and hunger will lead them to savage any unfortunate intruder. Alchemists, witches, and wizards occasionally capture Hydras for their magical experiments, as the beasts' rapid tissue replication makes them ideal subjects for tests meant to harness their regenerative powers. Some eccentric scholars claim that within the biological makeup of Hydras lies the secret to eternal life, though more sensible sages dismiss these hypotheses as absurd and unfounded.

Campaign Role

Hydras can fit into any adventure wherein PCs must pass through a lonely or desolate place or into a forgotten ruin or dank set of labyrinthine caves. Because of its high number of attacks, a Hydra is often miscast as a hard-core combat brute, but it actually fills the niche of an ambush monster much more effectively. Its animalistic intelligence-while entirely primitive-is equivalent to that of a dolphin or whale, and while it is not smart enough to be a villain per se, a Hydra is bright enough to employ strategies in combat that work to its advantage. This includes utilizing cover, such as by attacking creatures near the water's edge from underwater, or by trapping unwary foes in narrow bottlenecks separating one cavern from another. A Hydra knows that its body is more vulnerable to permanent damage than its multiple heads, so a particularly clever individual might hide its torso behind cover and attack with its necks exposed, distracting opponents from its body and making them concentrate on its perpetually regenerating heads. A Hydra also knows when it is advantageous to retreat and heal (including regrowing new heads) before returning to the attack, charging in and using its pounce ability. A hydra's lair should include varied terrain it can use to hide in, especially interconnected pools and bogs where it can submerge itself entirely and ward off fiery assaults or impede ranged attacks while it hides, heals, and prepares its next ambush from another direction.

Hydras are enticing and exciting monsters for the PCs to face near the end of low-level adventures. While the bestial intellects of Hydras do not make them viable fixtures for heroic PCs to pursue and destroy as harbingers of evil, the creatures' enormous size and unique combat style do provide for thrilling and dramatic random encounters. Conversely, Hydras can also be implemented as servitors for more powerful NPCs and villains, and are thus particularly well suited for the role of a monster leading up to the final fight in an adventure. A hydra's regenerative powers make it a viable recurring monster, as the creature might run away after taking a certain amount of damage or having so many of its heads cauterized, letting itself heal before once again taking on the PCs. The many variants make for interesting encounters should the PCs be in an area especially rife with the multi-headed creatures or should the PCs infiltrate the lair of a mad wizard who favors the Hydra as a subject for various kinds of experiments.

Treasure

As creatures with animal-level intelligence, Hydras have little interest in collecting treasure for the purposes of accumulating wealth; they do, however, dimly understand its usefulness as an attractant for prey, knowing that shiny trinkets and inedible objects are things some creatures prize enough to run heedlessly into danger to acquire them. As a Hydra consumes its prey, tearing it limb from limb between several jaws, it uses the teeth of its other heads to pry off items carried or worn, especially those that glitter and shine, and places these sparkling enticements around the fringes of its domain to be discovered by foolish passersby, hopefully emboldening them to explore the marshes, caves, and ruins where the Hydra lurks in waiting.

Hydras have learned to recognize common humanoid containers such as saddlebags for what they are, and when a Hydra devours a set of riders or stumbles upon a deserted carriage, it sends its many heads into the dark nooks and crannies of such vessels in search of hidden baubles and goods, which it may then drag into the light for display. Explorers are often puzzled by the valuables scattered haphazardly on the ground and hung from tree branches near a hydra's lair, still adorning the rotted or skeletal body parts of their former owners. Large or fragile items are often broken by the indelicate jaws of the Hydra, as it lacks hands to handle items carefully, yet as long as the rubble sparkles, the Hydra is content to display it. Drab items are ignored by a Hydra and left where they lie, usually close to the remains of their former owners.

Hydras in Mythology

The Hydra was an otherwise nameless water serpent in Greek mythology, spawned by Typhon and Echidna and raised by the goddess Hera to destroy Heracles. Called the Lernaean Hydra because of its lair (Lake Lerna near the city of Argos, a body of water fed by a sacred healing spring and said to cover a gate to the Underworld), the hydra's breath and blood were deadly poisonous, and even the spoor of its passage was lethal to those trying to track it.

Destroying the Lernaean Hydra was the Second Labor of Heracles, who used flaming arrows to flush the serpent out of its cave by the spring that fed the lake. He covered his face with a cloth to protect against the hydra's poison, but as he fought, each head he destroyed grew back as two. His nephew Iolaus came up with a plan to scorch each neck-stump with a burning branch to prevent it growing back, allowing Heracles to defeat it. Heracles used the hydra's tainted blood on his arrows thereafter; the poison was so potent it polluted the entire river where he killed Nessus the centaur and even brought death to Heracles himself when he donned a tunic soaked in Nessus's Hydra-contaminated blood.

Variants

Schism Hydra (+0 CR)

A schism Hydra possesses only three heads (but still has 5 Hit Dice), and its unique makeup makes it more akin to an ooze-like creature than a serpent-its body is capable of splitting into two identical copies. A schism Hydra does not possess the rules for sundering or regenerating heads, and instead gains the split (piercing or slashing, 9 hp) defensive ability. After combat, a schism hydra's strongest remaining body swallows the others in order to heal itself. A schism hydra's split ability is not affected by acid or fire as a normal hydra's is, and the only way to stop it is to drive it below the specified number of hit points.

Warden Hydra (+2 CR)

Warden Hydras are guardians of cultic sites and great treasures, either trained to protect such areas or simply drawn to the mystical energies therein. Their keen senses give them blindsense 60 feet and Blind-Fight as a bonus feat, and they automatically disbelieve illusions. Warden Hydras also have all-around vision and a racial bonus on Perception checks equal to their number of heads (rather than +2). A warden Hydra gains a +4 bonus to its Dexterity. Mind-affecting effects do not affect a warden Hydra unless they can affect as many targets as the Hydra has heads.

Grave Hydra (+3 CR)

These vile Hydras are the result of dark wizardry, imbued with necromantic energies that fill them with hatred and taint them with evil. A grave Hydra has an alignment of neutral evil, possesses negative energy affinity and DR 5/magic, and has an unnatural aura of 30 feet. In addition, a creature struck by a grave hydra's bite attack must succeed at a Fortitude save (DC 10 + 1/2 the hydra's Hit Dice + the hydra's Con modifier) or take 1 point of Constitution damage.

Miasma Hydra (+9 CR)

These powerful Hydras are found only in the most polluted or dangerous swamps. They have four legs and 12 heads, and their blood flows with a burning toxin. A miasma Hydra is immune to disease and poison, and its bite carries an agonizing venom (save Fort DC 10 + 1/2 the hydra's Hit Dice + the hydra's Con modifier; frequency 1/round for 6 rounds; effect 1d3 Str damage and sickened for 1 minute; cure 2 consecutive saves). A creature that sunders one of a miasma hydra's heads, deals bleed damage with a melee weapon, or confirms a critical hit with a slashing or piercing melee weapon is sprayed with its poisonous blood as if bitten. Each of a miasma hydra's heads has a breath weapon identical in effect to cloudkill (Fort DC 15 + the hydra's Con modifier for partial damage) that affects a 15-foot cone but with an instantaneous duration, usable every 1d4 rounds. A creature in the area of multiple breaths in the same round must make multiple saves, but can only be affected once per round.

A miasma Hydra gains Toughness and Snatch as a bonus feat, a +7 natural armor bonus to its AC from its additional heads, DR 10/cold iron, and a +2 bonus to all its ability scores except Intelligence.

Here is an example of a miasma hydra.

Hydra Larvae

The larvae of a Hydra can be just as imposing foes as the matured brute itself. The noxious eggs and writhing tadpoles are placed by their parent in shallow bog holes and trenches, and in these lairs, Hydra larvae prey upon those creatures that fail to watch their step as they cross the marshy terrain. Creatures that succeed at a DC 15 Perception check or Knowledge (nature) check notice Hydra larvae swimming in a bog hole. Hydra larvae can detect creatures outside of their pit, and burst from beneath the water to feast upon prey. Any creature within 5 feet of a larva-infested bog hole must succeed at a DC 15 Reflex save or become infested with Hydra larvae. A creature that becomes infested takes 1d2 points of Constitution damage and becomes sickened. Hydra larvae can be detached from a creature they dig into by prying them out with a slashing weapon (which requires a DC 20 Heal check that deals 1d6 points of damage regardless of whether or not the check succeeds) or by dealing acid damage to the larvae at any time, which deals half damage to the creature the larvae are infesting. Remove disease or a similar effect kills any Hydra larvae on the host.