BX Monsters A to Z: Hydra

 The hydra is a monster with deep roots in Greek myth. Like many D&D creatures portrayed as whole types or species, the hydra, too was once unique. It was a many-headed serpent, reputed to have poisonous breath and blood so toxic even the scent of it was deadly, with later incarnations of the myth adding the regeneration of two heads for each one severed. The slaying of the hydra served as the second of the mythic Labors of Hercules. 

The standard D&D hydra is toned down a bit from the myth. Though it's a tough beast to be sure, it can be slain by heroes of less than demigod status. The book describes it as having a dragon-like body (minus wings) and 5 to 12 snake-like heads. 

The hydra is a bit of an odd duck, mechanically. Its AC of 5 is decent. It has 1d8+4 heads, with 1 HD and 8 hit points for each one. That makes it quite a bit tougher than other monsters of the same HD, on average, since it always has maximum hp. Compare a 10-headed hydra with 80 hp to a typical 10 HD monster, which is about 85% likely to have between 35 and 55 hp. The number of heads also determines the number of bite attacks per round, each doing 1-10 damage on a hit. That can add up to a lot of damage: a 10-headed hydra against an AC 0 target will hit half the time, thus doing 5x1d10, or around 27.5 points of damage. 

Fortunately, dealing damage to the hydra will disable heads, one for each 8 hp inflicted. It's strangely reminiscent of how a dragon's breath weapon diminishes as the monster takes damage. If a party can get the jump on the hydra and whittle down its hit points before it can take a turn, they'll save themselves a ton of hurt. Of course, this 8-hp-per-head business can also cause some confusion with regard to area of effect attacks like fireball spells. It seems obvious, though, that players are not meant to be able to take out a hydra with a single 5-dice spell. The hit points aren't literally tied to each head as if it were a separate monsters; it's just a convenient abstraction to say that with each 8 points of damage a head is disabled.

Hydras aren't overwhelmingly fast, having the same movement rate as an unencumbered human, 120' (40'), but that's fast enough to keep up with a fleeing party, unless they're mounted or have speed-enhancing magic. With a morale score of 9, it's also pretty tenacious. Fortunately, under normal circumstances, you won't encounter more than a single hydra at a time, and usually, that's more than enough. 

The monster entry also makes note of a seagoing variant, which has fins instead of legs, but is otherwise statistically identical. With a dearth of monsters that make for interesting encounters in shipbound adventures, the sea hydra fills a definite need in the game.

The DM is also encouraged to create other variants to use as guardians for special treasures, such as a venomous hydra and a fire-breathing hydra that does 8 points per head. (No clarification given on areas of effect, or whether the breath is all one attack for a total of 8 points per head, or if each head can target separate opponents, or even if heads can concentrate their breath attacks in other proportions. Presumably, the usual saving throw for half damage applies, though.)

The description doesn't specify whether the monster is intelligent or not. Its Neutral alignment suggests an animal level of intelligence, but it saves as a fighter of level equal to its Hit Dice, not half, so I guess you just have to use your own judgment. Its Treasure Type is B, which is pretty mediocre, so unless it's placed as a guardian of a special treasure, its own hoard probably isn't enough to make up for the bother of killing it. 

Way back in the day, before I acquired the Expert Set, I recall that there was something in the Basic Set that encouraged DMs to create their own monsters, and the first monster I created was a version of the hydra, which was more directly inspired by the Lernaean hydra of Greek myth. Specifically, it had twelve heads, and two heads would regenerate for each one severed within a few rounds unless seared with a flaming brand. I don't recall exactly the stats I gave it, or how the mechanics of decapitation worked, but it used standard Hit Dice, rolling for hit points instead of an assigned 8 per head. I think it may have been a legless serpent, too, rather than the dragon-like physique. I remember being a bit disappointed with the official version, though with benefit of hindsight, it's actually not so bad, and probably easier to run at the table than whatever mess I had cobbled together.

Hydras can make for some superb encounters, both in the dungeon and outside it. A dozen heads breaking the surface of a stagnant marsh as the party rows across it toward some other destination? That's gold! A huge creature trapped in a room or passage behind a portcullis, but able to slither its serpentine necks through to attack passing characters? That'd be a fun one to spring on your players! You could use the same principle in a dense forest; adventurers can't evade the monster by weaving around trees; it can block them from multiple directions! Thematically, a hydra could fill in for other monsters to subvert cliches. A hydra could serve as the guardian of a labyrinth, or as the monster to whom villagers sacrifice virgins. Use a special hydra with a petrifying bite when a medusa or gorgon feels too overdone. Or give it aspects of different reptilian forms -- crocodile heads, with one floating on the surface of a body of water, and several others beneath the surface waiting to strike if the first misses its prey. Or make it a multi-headed snapping turtle, with a thick leathery shell for enhanced AC. If you want to stick with the classic serpent motif, make it a constrictor, using the same mechanics as the giant python. It's a monster with a ton of possibility.