Dragon, Black Mature Adult (CR 12)

Huge Dragon (Water)
Alignment: Always Chaotic Evil
Initiative: +0; Senses: blindsense 60 ft., darkvision120 ft., low-light vision, and keen senses
Languages: Draconic

AC: 29 (-2 size, +21 natural), touch 8, flat-footed 29
Hit Dice: 22d12+110 (253 hp); DR: 10/magic
Fort +18, Ref +13, Will +15
Speed: 60 ft., fly 150 ft., (poor), swim 60 ft.
Space: 15 ft./10 ft. (15 ft. with bite
Base Attack +22; Grapple +38
Attack: 1 Bite +28, 2 claws +23, 2 wings +23, 1 Tail Slap +23, 1 crush +23 melee; Breath +28 ranged
Damage: 1 bite 2d8+8, 2 claws 2d6+4, 2 wings 1d8+4, 1 tail slap 2d6+12, 1 crush 2d8+12, Breath weapon 14d4(26)
Special Attacks/Actions: Breath weapon, fear (DC 23), SR 21
Abilities: Str 27, Dex 10, Con 21, Int 14, Wis 15, Cha 14
Special Qualities: Acid immunity, water breathing, Darkness, Corrupt water CL 5th
Feats: #Feats: 8
Skills: Skill points: 50
Advancement: 23-24 HD (Huge)
Climate/Terrain: Any marsh and underground
Organization: solitary, pair, or family (1-2 and 2-5 offspring)
Treasure/Possessions: Double Standard

Source: Monster Manual

Breath Weapon (Su): A black dragon has one type of breath weapon, a line of acid. Using a breath weapon is a standard action. Once a dragon breathes, it can't breathe again until 1d4 rounds later. A blast from a breath weapon always starts at the dragon's mouth and extends in a direction of the dragon's choice. The black dragon's breath weapon deals 14d4 acid damage. Creatures caught in the area can attempt Reflex saves (dc 26) to take half damage

Water Breathing (Ex): The dragon can breathe underwater indefinitely and can freely use its breath weapon, spells, and other abilities while submerged.

Corrupt Water (Sp): Once per day the dragon can stagnate 10 cubic feet of water, making it become still, foul, and unable to support animal life. The ability spoils liquids containing water. Magic items (such as potions) and items in a creature's possession must succeed at a Will save (DC 23) or become fouled.

Other Spell-Like Abilities: 3/day - darkness (radius 70 feet).

Water Subtype

This subtype usually is used for elementals and outsiders with a connection to the Elemental Plane of Water. Creatures with the water subtype always have swim speeds and can move in water without making Swim checks. A water creature can breathe underwater and usually can breathe air as well.