The Sea Devil Earth was an alternate timeline in which the Sea Devils had conquered Earth shortly after being awoken at the Raven Peninsula in 1903 by the Skithra. (COMIC: Alternating Current) It came into existence due to a temporal paradox that occurred in 1969 due to an encounter between the Tenth and Thirteenth Doctors. (COMIC: A Little Help from My Friends)
This timeline deviated from the normal timeline in 1903. The Skithra, led by the Queen of the Skithra, were successful in kidnapping Nikola Tesla and also Thomas Edison. (COMIC: Alternating Current) In the original version of events, they had been thwarted by the Thirteenth Doctor and her friends. (TV: Nikola Tesla’s Night of Terror) The Skithra subsequently awoke the colony of Sea Devils at the Raven Peninsula so they could conquer Earth on the Skithra’s behalf.
By 2020, one Skithra Queen had turned against her people and was trying to help the humans fight back against the Sea Devils. The Thirteenth and Tenth Doctors both arrived in 2020 and allied with the rogue Queen and Rose to correct the timeline. Travelling back to 1903, they discovered what had changed and prevented the awakening of the Sea Devils and rescued the scientists, at the cost of the rogue Queen’s life. (COMIC: Alternating Current)