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The red leech — metaphorically known to the Eleventh Doctor as the "Crimson Horror" — was a species of leech native to Earth.
Biology
Red leeches were small creatures not dissimilar to lobsters with large black eyes, sucker-like mouths and two tentacles covered in barbs. They lived attached to other creatures, though they required excess salt to survive. If the host was dying, the red leech would leave and find a new one.
The red leeches could produce a deadly poison. Depending on the dosage, the poison could cause blindness, paralysis or death. In some cases, it would cause the person's skin to become red, waxy and hard. (TV: The Crimson Horror)
History
The red leeches were an ancient species; the Eleventh Doctor knew them to live in the Jurassic period, while Lady Vastra believed they died out 65 million years BC. They were known to the Silurians, who considered them their most virulent enemy.
The red leeches survived into the Victorian era and one such creature was discovered by Winifred Gillyflower who named it Mister Sweet. She created Sweetville, enticing people to move there and then paralysing them with red leech venom. She then planned to send up a rocket containing red leech venom, which would explode and shower the population with poison. The people of Sweetville could then be revived to repopulate the world. (TV: The Crimson Horror)
Behind the scenes
The red leech is a reference to "the repulsive story of the red leech", a Sherlock Holmes adventure mentioned by Dr. Watson in "The Adventure of the Golden Pince-Nez".