Sea Devil

The Sea Devils, once called the Aquatic Silurians, were amphibious Earth reptiles. Like their land-based cousins, the Silurians, they lived in prehistoric times in a scientifically-advanced civilisation.

As their aquatic colonies awoke from suspended animation, these one-time rulers of the Earth attempted to reclaim their planet, leading to conflict with the new rulers: humankind.

Biology
The Sea Devils were related to the Silurians, though they were genetically divergent enough to create problems for the offspring of such matings. (PROSE: The Scales of Injustice)

Like the Silurians, variations existed among the Sea Devils with different subspecies and appearances. Sea Devils typically looked somewhat like humanoid turtles or plesiosaurus. They had beak-like mouths, large eyes, two fins on their head and long necks. They had three webbed digits on their hands and feet. Their skin tones came in a variety of shades, including green, blue and orange. They were much tougher than humans, since they needed to survive the crushing depths of the ocean. Sea Devils communicated in a language like sonar, but they could speak human languages in a whispering, rasping voice. (TV: The Sea Devils) A Sea Devil's skin was slimy to the touch. (PROSE: The Silurian Gift)

One group of Sea Devil warriors, which appeared at Sea Base 4, were notably distinguished by their silver skin and their lack of head fins. (TV: Warriors of the Deep)

Another subspecies of Sea Devil, suggested by the Twelfth Doctor to be a "warrior-class", were larger, faster, and stronger than those that he was familiar with. Their muscular bodies sported dark green skin, small red eyes, a pronounced jaw, and a large tail. They lived on Ravenscaur Island. They could communicate using a form of sound-based telepathy and could use this ability as a weapon by producing a deadly sonic scream. (COMIC: Clara Oswald and the School of Death)

There also existed a class of Sea Devils who were much smaller, about the size of an infant human, that could merge with a human host, creating a human-Sea Devil hybrid but with the Sea Devil's psyche dominating its host. They possessed the sound-based abilities of their warrior-class counterparts but also displayed potent psionic powers, specifically telekinesis. The Twelfth Doctor explained that this was a side effect of two minds merging into one.

Culture
The Sea Devils worshipped the Great Old One known as Dagon as their god. (PROSE: All-Consuming Fire) They were determined to retake the planet from the humans. They held an unwavering view that, as the original rulers, it would always be their right to do so. (TV: The Sea Devils)

Technology
The Sea Devils were more advanced than the humans of the 20th century when they went into hibernation. They had colonies around the world, each filled with thousands of Sea Devils. (TV: The Sea Devils) The colonies could be awakened en masse through the use of a sonic signal. (TV: The Sea Devils) They built their underwater colonies inside caves. One base, located near a sea-fort, had simple grey-black-coloured walls, (TV: The Sea Devils) while another appeared to be carved into the rock, and was covered in algae and seaweed. (TV: Warriors of the Deep)

They also used special orange capsules to transport land-dwelling creatures. (TV: The Sea Devils)

The Sea Devils had stockpiles of weapons in these bases. They typically used small, round, side arms. (TV: The Sea Devils) During their invasion of Sea Base 4, they made use of a larger version of the device to cut through the doors. (TV: Warriors of the Deep)

The Sea Devils which attacked HMS Seaspite wore gauze-like dresses. (TV: The Sea Devils) The Sea Devils warriors in the 2084 invasion wore thick black armour reminiscent of samurai armour. (TV: Warriors of the Deep)

The Sea Devils that attacked Ravenscaur Island had an enormous underground colony located in a network of caves that artificially reproduced the environment and climate of prehistoric Earth, indicating they had advanced terraforming and environmental technology.

They had colossal war machines that were shaped like giant metal spheres with tentacles that could pierce through the hull of a human battleship easily.

Sea Devils also had large submarines heavily armed with laser armaments that could easily outmatch an aircraft carrier with its speed and firepower.

A fearsome piece of technology they possessed was sonic destructors. These were enormous tower-like structures that produced sonic pulses capable of sinking an entire island by causing intense earthquakes.

History
The Sea Devils ruled the Earth when humans were apes, just as the Silurians did. (TV: The Sea Devils) Both the Silurian and Sea Devil races were led by the Triad, their "custodians". (TV: Warriors of the Deep) When the Moon approached Earth, (TV: Doctor Who and the Silurians) the Silurian and Sea Devil scientists calculated a great catastrophe in which "all life" would be wiped out (TV: Doctor Who and the Silurians, The Sea Devils) by the Moon "sweep[ing] by Earth" (PROSE: Doctor Who and the Cave-Monsters) and drawing away the atmosphere. (TV: Doctor Who and the Silurians) The Third Doctor repeated the claim to Marc Marshall that the Silurians built the shelters when they thought the planet would "suck away" Earth's atmosphere as it rushed by. (PROSE: The Scales of Injustice) Conversely, the Fifth Doctor recalled that the Silurian astronomers predicted that Earth was about to be "struck" by the planet at the time, (PROSE: Warriors of the Deep) and the Eleventh Doctor described the trajectory of the planet the astronomers had predicted as a "crash course". (TV: Cold Blood) The catastrophe did not happen as predicted, and the Sea Devils remained inside their hibernation units. During millions of years of hibernation, the reactivation machinery of the hibernation units deteriorated. (TV: The Sea Devils)

In 1909, a group of Sea Devil woke up beneath Raven Peninsula. They used terraforming technology to create an earthquake which separated the peninsula from the main land. Ravenscaur became known as Raven's Isle. The Sea Devils infiltrated the school and village on the island by turning the humans into Sea Devils. (COMIC: Clara Oswald and the School of Death)

In 1972 or 1982, (COMIC: Clara Oswald and the School of Death) a group of Sea Devils hibernating beneath the English Channel were awoken by. He attempted to persuade them to reclaim the planet from the human race. Despite the Third Doctor's efforts to convince them otherwise, hostilities escalated when Private Secretary Walker ordered the Royal Navy to attack the base, and the Sea Devils decided to go to war with the humans. However, before they could use the Master's sonar device to reactivate more bases, the Doctor blew up their base by overloading the devices by reversing the polarity, preventing the war between humans and Sea Devils from starting. (TV: The Sea Devils)

In 1983, Sarah Jane Smith and K9, visiting Warren Martyn's archaeological expedition in Egypt, discovered an underground city of Silurians and Sea Devils. Sarah Jane opened diplomatic relations with them. On her return to England, she contacted the Brigadier, who wished to make amends for his earlier mistakes. (COMIC: City of Devils)

In the early 21st century, a colony of both Silurians and Sea Devils were discovered under the South Pole. After a corrupt businessman attempted to blackmail them into providing him with a unique energy source, they revolted, with the Sea Devils launching an attack. They were forced back into hibernation by the Eleventh Doctor, who collapsed the crevice leading to their colony afterwards. (PROSE: The Silurian Gift) A colony of Sea Devils were awoken in 2084 by the Silurians. They were sent as shock troops to invade Sea Base 4. Though they were better prepared, they were all killed by the Hexachromite gas used by the Fifth Doctor. (TV: Warriors of the Deep)

By the 26th century, the inter-species conflicts appeared to have come to an end. Silurians and Sea Devils were accepted into Earth society, and the term "Earth Reptile" was apparently popularised among humans. (PROSE: Love and War)

In the 30th century, the Earth Reptiles were part of the Earth Empire and were considered full members of that government as an Earthborn species. (PROSE: So Vile a Sin) In the 32nd century, it became illegal to trade in Earth Reptile pituitary analogue extract on the planet Dramos. (PROSE: Burning Heart)

Around the 26th century, a number of Sea Devils and humans colonised the planet Chosan, which was being attacked by Calabraxian terrorists. The Earth Reptile Council asked the Braxiatel Collection for aid. They sent Bernice Summerfield. She found the ocean planet held a sentient protein in the waters which had began to infect the water-dwelling Sea Devils.

The entities attempted to possess the Sea Devils and use them as an army against the humans but faced difficulty in integrating themselves into the host's biology. This led to mutations. They took the Sea Devil Lurnix as a host. They tried to have Lurnix lower the security grid and infect all the Sea Devils for an army.

However, Bernice Summerfield evacuated the colony and detonated a bomb at Chosan, ending that threat. The Sea Devils were forced to abandon the Chosan colony due to the continuing threat. (AUDIO: The Poison Seas)

In the 101st century, the Earth Reptiles were part of the Union. (PROSE: The Crystal Bucephalus)

Undated events
During the Last Great Time War, a Sea Devil was imprisoned by an unknown force. (COMIC: The Forgotten) A group of Sea Devils were imprisoned in the TARDIS by the Master. (GAME: Destiny of the Doctors) A Sea Devil was worth an unidentified amount of points in the I-Spyder Book of Earth Creatures if it was spotted. (PROSE: The Last Dodo)

As shown in the Anubian Book of Deliverance, the Anubians attacked the Sea Devils. (TV: Curse of Anubis)

A Sea Devil visited the beach on Beachelguese with a Silurian and a Macra. (COMIC: Surfshock)

Alternate histories
In one alternative version of the 1990s, the Doctor having died during the Wenley Moor incident, the Silurians ruled Earth unchecked while the Sea Devils served them as underwater soldiers. (PROSE: Blood Heat)

In an alternate timeline created by the Black Guardian where the First Doctor never left Gallifrey, and became Lord President, the Sea Devils and the Silurians fought over the Earth along with several species of alien invaders. This timeline was destroyed when the Seventh Doctor retrieved the Key to Time. (COMIC: Time & Time Again)

In an alternate dimension in which the Doctor's adventures existed as a fictional TV show called Doctor Who, a fan dressed as a Sea Devil was present at a Doctor Who convention which the Eleventh Doctor visited with Ally in 2013. (COMIC: The Girl Who Loved Doctor Who)

Behind the scenes

 * A Sea Devil was to have appeared as one of the dead prisoners on the spaceship in The Stones of Blood. Although Malcolm Hulke was paid a copyright fee, shots of the creature were lost at the editing stage.
 * The Alan Moore graphic novel series refers to lizard-men called Silurians and Sea Devils and retcons  as one of the latter.
 * The Sea Devil and Silurian are shown as the original inhabitants of Earth in the steampunk graphic novel.