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 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 already very large creatures, but could grow to truly immense size.

Personality
Wastriliths 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.

Abilities
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 breathe 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 by 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. In addition to the powers common to most demons, they could cast control water, deeper darkness, fear, greater dispelling, read magic, suggestion, telekinesis, teleport without error, tongues, 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, as well as summon monster IX up three times every day, though the latter power was limited to summoning aquatic monsters.

Combat
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. The first thing they did in combat was summoning more minions, before they started using their other magical abilities against their foes. Those that learnt magic through wizardly study naturally specialized in elemental water magic.

Society/Ecology
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, fighting for superiority even amongst themselves, but at the same time concerned themselves primarily with their own business. 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, instead simply putting holes inside coral reefs for themselves to live in.

They were normally found in the aquatic environs of the Abyss and other lower planes such as the Fated Depths or the River Styx, the effects of which they were immune to. A vast number of water lords made their homes in the cold, dark regions of Demogorgon's Gaping Maw, the 88 layer of the Abyss. Wastriliths were also the favored minion of Dagon, as the water lords had once been obyriths themselves before evolving into their own separate demon type, and were given free reign over the Shadowsea so long as their predations didn't interfere with demon lord's inscrutable agenda.

Despite not needing to eat, wastriliths 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.

Blood War
It often fell to the wastriliths to conduct the marine affairs of the Blood War, gathering aquatic entities to repel sea-faring invaders and traveling through the River Styx to make offensive strikes against Hell. They delighted in capsizing Baatorian ships in order to cause legions of lesser devils to fall into the river, thus giving them all amnesia and inducing chaos in their ranks. No one actually forced them to fight, their warring being an extension of their fight for supremacy in the waves of the lower planes.