Styx Dragon

(From Planescape: Monstrous Compendium Appendix II - 1995):

There are few things in the Lower Planes that even the fiends're afraid of, and the shadowdrake is one of 'em. Also known as death drakes, Styx dragons, or darkwyms, shadowdrakes are relatives of dragonkind who inhabit the upper reaches of the fiendish planes. They commonly prey on the weaker ranks of fiends around them, but they'll devour travelers or careless high-up fiends just as quick. Unlike many true dragons, shadowdrakes've got no interest in conversation, servants, or plots; every sod they meet's just another meal waiting to be eaten.

Shadowdrakes have long, serpentine bodies covered with slimy scales ranging from dark brown to rusty red. Their eyes glow with a feral yellow light, and mismatched fangs just from their terrible jaws. The wings of a shadowdrake are vestigal, and can't support the creature in flight. Instead, the drake's wings are used with its powerful tail for swimming. Shadowdrakes are one of the few creatures that can abide the touch of the polluted Styx, and they frequently attakc sods on or near the river. On land, the shadowdrake's forced to slither snake-wise somce ots tomu legs can't supoort its weight.

The distinguishing feature of a shadowdrake is its twin tail, which forks about halfway down its length. Long, razorsharp blades of bone line the tips of each limb, and the shadowdrake can slash and grapple with its tails as well as other dragon-kin can attack with their claws.

Combat: Shadowdrakes are extremely aggressive, territorial, and predatory. They're often overconfident, and they'll take on anyhthing smaller than a pit fiend or balor that comes anywhere near their lair or hunting ground. The shadowdrake attacks with a bite and two lashes from its tail; it's long and flexiible enough to use its forked tail against opponents in front of itself. The shadowdrake prefers to attack with a sudden rush from a dismal burrow or from the filthy waters of the Styx; if it can do this, it imposes a -2 penalty on its enemies' surprise checks.

Any sod injured by the drake's bite or tail lash is 50% likely to contract a gangrenous disease, even if [they survive their] wounds. This disease sets in within 3 to 24 hours, and renders the victim completely helpless with fever and delirium. If untreated, the rot reaches the victim's vital organs and hells [them] within 12 to 48 (d4x12) hours after onset. The disease can be cured by any spell or item that can cure disease or neutralize poison, but a character with the healing proficiency suffers a -4 penality to [their] proficiency check when treating the diease, and any cure wounds spell heals only half the normal points of damage if the gangrene is active.

Breath Weapon/Special Abilities: The shadowdrake's hesitant to use its breath weapon, since it usually deprives the beast of its intended meal. The drake expels a gout of sticky, corrosive spittle 30 feet long and 3 feet wide. Normally, it can strike only 2 or 3 targets at most, and then only if they're standing close together. If the victim makes [their] saving throw, [they take] half the listed damage. If [they fail], the corrosive goo causes full damage in the first round, half damage in the next round, and one-quarter damage in the third round. Leather, bone, or wood armor's destroyed by one round of contact; metal chain or scale's destroyed in two rounds; and metal plante is ruined in three rounds. (Magical armor or equipment gains an item saving throw versus acid to avoid the effect.)

A shadowdrake doesn't normally cast spells, but it does possess several spell-like abilities, cumulative with age, as shown below:

Hatchling = Immune to all poison or disease

Young = corrupt water by contact

Adult = darkness 15' radius 2/day

Old = hold monster 1/day

Wyrm = summon aquatic monsters 1/day

Habitat/Society: Shadowdrakes are most often found along the banks of rivers or in the swamps and bogs of the Lower Planes, including the Styx in all of its wanderings. They often dig extensive burrows of dank, slimy tunnels. Some portions of their lairs can be reached only by a long, black swim through a water-filled siphon. Shadowdrakes prefer to sleep in their lairs by day and hunt by night, sometimes traveling quite a distance from their lair before returning.

Shadowdrakes are territorial and don't get along well with each other. The only time two of these creatures'll be even together is during a rare mating season. Even then, the drakes don't remain together long or cooperate in hunting or the defense of their young or their lair.

It's almost impossible to deal with a shadowdrake, because shadowdrakes don't listen to anything a weaker creature has to say, and they avoid contact with more powerful monsters. However, they're not stupid, and a drake'll attempt trickery or negotiation if physical methods clearly aren't going to work. Shadowdrakes are cunning and deceitful; they love to twist words or make false promises whe n they see the chance.

Ecology: The bulk of a shadowdrake's diet comprises the most wretched types of fiends, such as lemures, dretches, nupperibos, or manes. Powerful shadowdrakes've been known to take hydroloths or marrenoloths, and it's not at all uncommon for any solitary traveler to atrract the attention of a shadowdrake.

(From 3.5e Draconomicon: The Book of Dragons - 2003):

Also known as shadowdrakes or darkwyrms, Styx dragons haunt the putrid waters of the River Styx throughout its nearly infinite length. One of the few creatures immune to the harmful effects of the river, Styx dragons swim with impunity across the top layers of all the Lower Planes, feasting on fiends and any other creature they can find and catch. 

A Styx dragon has a long, serpentine body with tiny, flipperlike claws that are useless on land and in combat. Its wings are too small to carry it aloft, but help to propel it through the water. Its tail splits into two long, bladed whips that it can use to slash and grab its prey. A Styx dragon’s scales are slimy and range from dark brown to rusty red in color. Its eyes glow with a lurid yellow light. Styx dragons make their lairs by burrowing into the mud on the banks of the Styx. They do not like to leave their native plane, but if one is forcibly brought to the Material Plane, it will thrive in fetid water. 

Styx dragons normally subsist on the flesh of fiends, but enjoy eating any meat— particularly rotting carrion. 

Combat: A Styx dragon’s physical attacks are limited: It attacks with the twin blades of its tail instead of claws, it cannot use its wings effectively, and it does not gain tail slap attacks in addition to its tail blades. Its tail blades deal damage as if the dragon were one size category larger than actual, however, as does its tail sweep. 

Young and older Styx dragons’ natural weapons are treated as magic weapons for the purpose of overcoming damage reduction. 

Breath Weapon (Su): A Styx dragon has two types of breath weapon, a line of acid that persists for 3 rounds (dealing half damage on the second round and one-quarter damage on the third round), or a cone of stupefying gas. Creatures within the cone must succeed on a Fortitude save or take 1 point of Intelligence damage per age category of the dragon. 

Amphibious (Ex): Although Styx dragons are aquatic, they can survive indefinitely on land. 

Constrict (Ex): With a successful grapple check, a Styx dragon can crush a grabbed opponent, dealing twice its tail blade damage as bludgeoning damage. 

Disease (Ex):Any creature hit by a Styx dragon’s bite or tail attack must succeed on a Fortitude save (DC equal to that of the dragon’s breath weapon save DC) or contract Stygian wasting. The symptoms of the disease include flesh rotting away and hair falling out. The incubation period is 1 day, and the disease deals 1d6 points of Charisma damage. A victim must make three successful Fortitude saves in a row to recover from Stygian wasting (see Disease, page 292 of the Dungeon Master’s Guide). 

Improved Grab (Ex): To use this ability, a Styx dragon must hit a creature that is at least one size category smaller than itself with its tail blade attack. If it gets a hold, it can constrict in the same round. It can also attempt to start a grapple as a free action without provoking an attack of opportunity. 

Spell-Like Abilities: At will—curse water; 3/day—control water, deeper darkness, fog cloud; 1/day—feeblemind, hold monster, horrid wilting, mind fog, stinking cloud. 

Styx Adaptation (Ex): Styx dragons are immune to the harmful effects of the River Styx, and they can breathe water. 

Summon Monster VIII (Sp): Once per day, a great wyrm Styx dragon can summon a fiendish giant squid, 1d3 fiendish giant octopi or Large tojanidas, or 1d4+1 Large water elementals, Huge fiendish sharks, or fiendish giant crocodiles. Aside from the monsters available, this ability is identical to summon monster VIII. Caster level 15th.