Wastrilith

Wastriliths, otherwise known as water lords, were demons found in the aquatic environments throughout the Lower Planes. They corrupted nearby water sources with the Abyss's fell power, allowing them to control it to serve them.

"The corruption left behind when a wastrilith visits the world can persist for decades. If left unchecked it can become a bridge to the Abyss."

- Mordenkainen

Description
Wastriliths were 10 feet (3 meters) long creatures resembling segmented eels with bulging, unblinking eyes, and mouths full of pin-like fangs. A powerful tail attached to their humanoid upper body, with arms ending in claws.They were generally very big creatures but could grow into truly immense size.

Personality
Watriliths were most easily described as arrogant bullies. They intimidated others into submission and terrorized their environment among others awful activities. Their influence could be especially dangerous when they encouraged people of evil temperament to become pirates and other criminals.

Combat
Although wastriliths could move on land, they were generally faster swimmers, and better combatants in water. They fought with their claws and fangs, but could also breath boiling water to attack their prey. Their complete immunity to cold temperatures was contrasted by their susceptibility to hot temperatures. They were master manipulators of water, corrupting it simply be being near it, able to alter currents to block or move their enemies, and being immune to any effects involving it, including their own boiling water attacks. Those who drank from water corrupted by a wastrilith found it to be incredibly poisonous, maddening those that it did not kill. This effect did not apply to other demons, who found their foul water restorative to drink.

They had a large number of magical abilities. They could cast blasphemy, control water, deeper darkness, desecrate, detect good, detect law, fear, greater dispelling, read magic, suggestion, telekinesis, teleport without error, tongues, unhallow, unholy blight, and wall of ice as often as they wanted. Their ability to cast teleport without error was limited to themselves and up to 50 pounds (22.7 kilograms) of goods, and their ability to cast tongues was also limited to casting on themselves. Other magical abilities of theirs included the ones to cast any of the symbol spells, unholy aura, and summon monster IX up three times every day. Their ability to cast summon monster IX was limited to summoning aquatic monsters, but it was generally the first thing they did, before they started using their other magical abilities against their foes.

Wastriliths saw no reason to unnecessarily put themselves into danger when they had subjects to do that for them. Unless all of their minions were defeated they were content to let battles unfold without intervening.

Society
Despite not having magical control over undersea creatures, the reputation of wastriliths was so well known that most, even summoned water elementals and animals did their bidding. They were fiercely territorial but at the same time concerned themselves primarily with their own business. A wastrilith was usually found alone, though it tried to summon aid as the first thing to do in the case of combat. As mentioned above, they enjoyed claiming the fealty of others and built themselves elaborate underwater castles when found on the Prime Material plane. Their domains were circles of approximately 25 miles in diameter, with the castles in the center. Some did not create marvelous sanctums but simply put holes inside coral reefs for them to live in. They were normally found in the aquatic environs of the Abyss and other Lower planes such as the Fated Depths. A vast number of water lords made their homes in the cold, dark, 88th layer of the Abyss  known as the Maw of Demogorgon.

Despite not needing to eat they nonetheless indulged themselves in the flesh of intelligent marine life like merfolk and aquatic elves. The remains were often left to dissuade others from entering their territories, as was the sinking of ships and driving of sea dragons and other powerful undersea life.

A wastrilith could be summoned with a summon monster IX spell, but doing so was potentially lethal for the summoner for a wastrilith could try to break its bonds to the summoner and turn on him or her.