The cousins of another race called the Silurians The Sea Devils are a Reptile Race who Lived on the Planet Earth before Humans did. They Ruled the Oceans as the Human Race were still Apes. They went into Hibernation and then woke up in the 20th Century. Their Cousins are another Reptilian Race which The 3rd Doctor has Faced the Silurians. These Sea devils wore Net Clothing which looks like Flotsam and Jetsam from the Ocean. Their Weapons were Circular Flash Guns so they could kill more Quicker. When they found out that the Humans Ruled Earth now they then Rose from the Waters and were going to Invade and take back their Planet Earth. They had a Villain helping them who escaped from an Island Prison not far away The Master. The Sea Devils were told by The Doctor to Live in Peace with the Humans but The Master wanted them to go to War against the Humans as they killed a lot of them already. Then the Sea Devils put The Doctor and Master to their Prison Chamber. They Escaped but The Doctor had blown up the Sea Devil Colony.
Over 100 Years Later the Sea Devils woke up again working as Foot Soldiers for their Cousins the Silurians which they wanted to Provoke War between two of the Human Power Blocs. Once the Humans were weakened by the War the Sea Devils and Silurians had then invaded an Underwater Station at the Bottom of the Sea. The Sea Devils wore Kitted War Armor, they had Improved Circular Flash Guns and when they died Green Blood came out of them. That's when they wanted to take back their Planet of Earth by Invasion. But the 5th Doctor had to kill the all of the Sea Devils and he did. Then when the Myrak was Killed then the Silurians Died. The Doctor had Saved Earth from 2 Reptilian Families.
They have appeared in the 3rd Doctor Season 9: The Sea Devils
5th Doctor Season 21: Warriors of the Deep.