Dragon, Styx Mature Adult (CR 15)

Huge Dragon (Extraplanar)
Alignment: Always neutral evil
Initiative: +0; Senses: blindsense 300 ft., darkvision 1000 ft., and low-light vision
Languages: Draconic

AC: 30 (-2 size, +22 natural), touch 8, flat-footed 30
Hit Dice: 23d12+115 (264 hp); DR: 20/magic
Fort +18, Ref +13, Will +17
Speed: 60 ft., burrow 20 ft., swim 60 ft.
Space: 15 ft./10 ft. (15 ft. with bite)
Base Attack +23; Grapple +40
Attack: 1 bite +30, 2 tail blades +25, 1 tail sweep +25
Damage: 1 bite 2d8+9, 2 tail blades 2d8+4, 1 crush 2d8
Special Attacks/Actions: Breath weapon 7d6 (26)
Abilities: Str 29, Dex 10, Con 21, Int 18, Wis 19, Cha 18
Special Qualities: Amphibious, immune to poison and disease, constrict, improved grab, Styx adaptation, Curse water, fog cloud, Deeper darkness, stinking cloud, SR 25, Fear aura (DC 25)
Feats: #Feats: 8
Skills: Skill points: 98
Advancement: 24-25 HD (Huge)
Climate/Terrain: Any aquatic (lower planes)
Organization: Solitary (1 dragon, any age), clutch (1d4+1 wyrmlings, very young, young, juvenile, or young adults), family (pair of mature adults and 1d4+1 offspring)
Treasure/Possessions: Triple Standard

Source: Draconomicon

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

Breath Weapon (Su): A Styx dragon has two types 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 point of Intelligence damage per age category the of 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 grappled 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 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.)

Improved Grab (Ex): To use this ability, a Styx drag on 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.

Tail Sweep: A Styx dragon of Huge size can sweep with its tail as a standard action. The sweep affects a half circle with a diameter of 30 feet, centered on the dragon's rear. Creatures within the swept area are affected if they are four or more sizes smaller than the dragon. Creatures can attempt Reflex saves to take half damage against a DC 26.

Frightful Presence (Ex): A mature adult dragon can unsettle foes with its mere presence. The ability takes effect automatically whenever the dragon attacks, charges, or flies overhead. Creatures within a radius of 210 feet are subject to the effect if they have fewer HD than the dragon (23).

A potentially affected creature that succeeds at a Will save (DC 25) remains immune to that dragon's frightful presence for one day. On a failure, creatures with 4 or fewer HD become panicked for 4d6 rounds and those with 5 or more HD become shaken for 4d6 rounds.

Dragons ignore the frightful presence of other dragons.

Immunities (Ex): All dragons are immune to sleep and paralysis effects. Styx dragons are also immune to acid, poison and disease.

Spell Resistance (Ex): As dragons age, they become more resistant to spells and spell-like abilities.

Blindsight (Ex): A dragon can ascertain creatures by non-visual means (mostly hearing and scent, but also by noticing vibration and other environmental clues) with a range of 210 feet.

Keen Senses (Ex): A dragon sees four times as well a human in low-light conditions and twice as well in normal light. It also has darkvision with a range of 700 feet and scent ability to 210 feet.

Extraplanar Subtype

A subtype applied to any creature when it is on a plane other than its native plane. A creature that travels the planes can gain or lose this subtype as it goes from plane to plane. This book assumes that encounters with creatures take place on the Material Plane, and every creature whose native plane is not the Material Plane has the extraplanar subtype (but would not have when on its home plane). An extraplanar creatures usually has a home plane mentioned in its description. These home planes are taken from the Great Wheel cosmology of the D&D game (see Chapter 5 of the Dungeon Master's Guide). If your campaign uses a different cosmology, you will need to assign different home planes to extraplanar creatures.

Creatures not labeled as extraplanar are natives of the Material Plane, and they gain the extraplanar subtype if they leave the Material Plane. No creature has the extraplanar subtype when it is on a transitive plane; the transitive planes in the D&D cosmology are the Astral Plane, the Ethereal Plane, and the Plane of Shadow.