Sea slugs (Opisthobranchia alucinor) are a race of slug-like creatures which are able to control the minds of humans by latching on to them. They are ruled by the Mother Mallum.
The sea slugs are first met in the Sea Slug Quest, while investigating the disappearances of Kent and Kennith. The sea slugs were somehow caught by the fisherman on the Fishing Platform and brought aboard. The sea slugs then attached themselves onto the fisherman and possessed them. Luckily Kent and Kenneth got away before being possessed also.
The next step in their plans was to free their leader form the prison in which the Temple Knights had sealed thousands of years ago. How they arrived at Witchaven is uncertain. Mayor Eustace Hobb announced he'd be renovating the shrine to the west. Soon villagers began to become possessed and the Temple Knights of Falador wanted to know why. Soon an adventurer discovered their plans of releasing Mother Mallum but was unable to stop them. However the adventurer managed to kill the slug prince before barely escaping Mother Mallum's possession attempt.
As for the rest of the population of Witchaven they were possessed and the town became the first in RuneScape to fall to the Sea Slugs.