Dragon, Black Great Wyrm (CR 21)

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

AC: 42 (-4 size, +36 natural), touch 6, flat-footed 42
Hit Dice: 37d12+296 (536 hp); DR: 20/magic
Fort +28, Ref +20, Will +25
Speed: 60 ft., fly 200 ft., (clumsy), swim 60 ft.
Space: 20 ft./15 ft. (20 ft. with bite)
Base Attack +37; Grapple +62
Attack: 1 Bite +46, 2 claws +41, 2 wings +41, 1 Tail Slap +41, 1 crush +41, 1 Tail sweep +41 melee; Breath +46 ranged
Damage: 1 bite 4d6+13, 2 claws 2d8+6, 2 wings 2d6+6, 1 tail slap 2d8+19, 1 crush 4d6+19, 1 tail sweep 2d6+19, Breath weapon 24d4(33)
Special Attacks/Actions: Breath weapon, fear (DC 41), SR 28
Abilities: Str 37, Dex 10, Con 27, Int 20, Wis 21, Cha 20
Special Qualities: Acid immunity, water breathing, Darkness, Corrupt water; Plant growth; Insect plague, Charm reptiles; CL 15th
Feats: #Feats: 13
Skills: Skill points: 191
Advancement: 38+ HD (Gargantuan)
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 24d4 acid damage. Creatures caught in the area can attempt Reflex saves (DC 36) 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 33) or become fouled.

Charm Reptiles (Sp): The dragon can use this ability three times per day. It operates as a mass charm spell that works only on reptilian animals. The dragon can communicate with any charmed reptiles at though casting a speak with animals spell.

Other Spell-Like Abilities: 3/day - darkness (radius 120 feet) and insect plague; 1/day - plant growth.

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.