Wastrilith

A Wastrilith was a demon found in aquatic environments.

Description
A wastrilith looked like a segmented eel with a powerful tail attached to a clawed humanoid upper body, bulging eyes, and a mouth full of fangs. They were generally very big creatures but could grow into truly immense size.

Personality
Watriliths were most easily described as arrogant bullies. They bullied others into submission and terrorized their environment, among others by encouraging people of evil temperament to become pirates and other criminals.

Combat
Wastriliths fought with their claws and fangs, but could also breath boiling water to attack their prey. They were completely immune to cold temperatures, but were susceptible to hot temperatures, but their complete immunity against any water based effect made them immune to their own breath of boiling water.

Wastriliths could walk on land, but were generally faster swimmers. They were also better combatants in water than on land.

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.

Society
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, though the were normally found in the aquatic environs of the Abyss and other Lower planes. They were also found on the Fated Depths.

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.