Sea Devil

The Sea Devils are a race of amphibious Earth Reptiles. The Sea Devils, like their land-based cousins the Silurians, were scientifically advanced, and lived during prehistoric times in an aquatic civilization. They are easily cowed into working with anyone to take back Earth, though they generally work with the Silurians best.

Appearance
Sea Devils look somewhat like humanoid turtles or plesiosaurus. They have beak-like mouths, large blue eyes, two fins on their head, a long neck and three digits per hand. They are much more lizard-like than the Silurians and don’t have a third eye. They come in a wide variety of blue, green and orange shades for skin tone. They are much tougher then humans, since they need to survive the crushing depths of the ocean. Sea Devils communicate in a language like sonar, but they can speak human languages in whispering tones. (DW: The Sea Devils)

The Silurians are closely related to the Sea Devils, though they are far enough apart genetically to create problems for the offspring. (DW: The Scales of Injustice)

Technology
The Sea Devils are reasonably advanced, but greatly rely on the advances of the Silurians. For example, they used the Myrka, a Silurian creation, when invading the sea base in 2084. (DW: Warriors of the Deep)

The Sea Devils have large underwater colonies, filled with Sea Devil soldiers. These bases appear to be carved out of rocks and a generally covered in algae and seaweed. There are also special orange capsules which can transport land-dwelling creatures. They have a large supply of weaponry in these bases and usually store thousands of hibernating soldiers. These colonies can be reactivated on mass, usually with a sonic signal. The Sea Devils commonly use a small, round, hand-held weapon. (DW: The Sea Devils)

Common Sea Devils wear a type of blue, gauze-like dress. (DW: The Sea Devils) The soldiers in the 2084 invasion, being elite soldiers, wore thick black armour somewhat reminiscent of samurai armour. (DW: Warriors of the Deep)

History
While the Silurians ruled the Earth, the Sea Devils were used as foot soldiers. When the Moon was coming dangerously close to Earth, they also went into their colonies to hibernate. After millions of years, some of the colonies were awaken.

In their first appearance they were awakened from their hibernation by the renegade Time Lord known as the Master. He attempted to persuade them to reclaim the planet from the human race. Despite the Doctor's efforts to convince them otherwise, the Sea Devils eventually decided to go to war, and the Doctor was forced to destroy their base. (DW: The Sea Devils)

The Sea Devils next appeared in 2084. This time, they were used by the Silurians in a second attempt to reclaim Earth from the humans. Though they were better prepared, they were all killed by the Hexachromite gas used by the Doctor. (DW: Warriors of the Deep)

Curiously, in Warriors of the Deep, the creatures used the terms "Silurians" and "Sea Devils" to refer to themselves, despite these terms previously being purely human phrases. This may be translation from their true names by the TARDIS.