Sea Devil

The Sea Devils are an intelligent species of amphibious reptilian humanoids which inhabited the Earth's oceans in huge underwater cities during prehistoric times, being contemporary to their genetic cousins, the land-based Silurians. Like the latter, they also went dormant en masse at some point in the Cenozoic, in order to avoid a natural disaster that could otherwise destroy them.

Individual colonies of Sea Devils would eventually wake up millions of years later than expected to find their home planet dominated by a newly evolved species: Humans, with whom they went in conflict at least twice: once in 1972 and again in 2084. Both times war was prevented by the Doctor's interfering with the Sea Devils' plans.

By the 26th century the Silurians and Sea Devils eventually emerged from their dormant colonies and were fully integrated in Human society, in which they were collectively known by the term "Earth Reptiles".

Biology
Sea Devils are generally regarded as reptiles, distantly related to the Silurians. They are tailless humanoids with an erect posture, taller than the average Human and capable of living just as well underwater as on land. Their faces somewhat resemble a turtle's, with a wide beaked mouth and a pair of relatively big eyes. They have robust necks, though skin, and speak in a raw whispering voice when on land, while using a sonar-based language undersea. In their first serial they also had a pair of long fin-like structures on the sides of their heads, but this feature is not present in the Sea Devils seen in Warriors from the Deep. They and the Silurians are both deadly vulnerable to hexachromite gas, which in large quantities causes their bodies to partially dissolve, leaking green fluids. This seems to imply that the Sea Devils have green blood.

Culture and Society
They live on cities constructed on the ocean floor, which nowadays are mostly covered by marine fauna and flora, making them hard to locate. In prehistoric times they lived alongside the Silurians and were close allies of them, acting as a warrior caste of sorts which voluntarily served their land-based relatives.

Religion wise they are known to worship Dagon

Appearances

 * Doctor Who season 9 (Third Doctor) - The Sea Devils (1972)
 * Doctor Who season 21 (Fifth Doctor) - Warriors of the Deep (1984)
 * Blood Heat, by Jim Mortimore (1993)
 * The Scales of Injustice, by Gary Russell (1996)
 * Big Finish's Bernice Summerfield series - "The Poison Seas" (2003)