Mother Mallum

Mother Mallum, originally known as Mahey'ish Medron (Jorral calls her both Mahey'ish Medron and Mahey'sh Medron, probably a typo), is the queen of the sea slugs and had plans to take over Gielinor. By the time players begin the Slug Menace quest, she already had control of most of the Fishing Platform and most of Witchaven. She, when her followers manage to free her from her prison, plans to take control of all important humans on the planet to achieve world domination, as she did the last time she was free.

She first revealed herself to the world when the Slug Menace Quest was released. Several thousands of years ago, likely during the God Wars, she emerged from the ocean. She tried to take over the world by taking over important political figures. However, she was stopped by the Temple Knights, who sealed her behind a magical door. Unfortunately, in Slug Menace, the player is tricked into opening that door using some specially crafted runes.

It appears that she communicates with an empty human body and during the last part of the Slug Menace quest, she tries to obtain the body of the adventurer who had inadvertently freed her. Fortunately, at the last minute, the adventurer was saved by Savant, who teleported him/her out of the cave.

Salt in the Wound
She is seen again during Salt in the Wound. A citadel has appeared in the middle of the Fishing platform and has Slug thralls guarding the entrance. The player, after enlisting help from Kennith (who looks notably much older), Ezekial Lovecraft and Eva, collect ingredients to produce a Anti-mind control serum and then go to the Citadel to confront the Mother Mallum herself, using each character's special abilities and teamwork. During the confrontation, she discards her host body and temporarily takes control of the player's body. However, the player's will is too strong to take over entirely and they are still able to give commands to the others. Using the Anti-mind control serum on Mother Mallum, she releases the player, then Eva squashes Mother Mallum with a pillar, freeing the Witchaven villagers and ending the ancient threat of the Sea Slugs.

Trivia

 * In the first book of the Wardstone Chronicles series, there is a witch called "Mother Malkin" who is a witch that can slip through cracks, holes, etc like a slug.
 * Mother Mallum and Captain Marlin may have a link, and possibly a team, as Captain Marlin has a slug on his head. Captin Marlin chathead.png
 * The name "Mallum", translated from Latin, means "of evil", thus making her, quite appropriately, the mother of evil.
 * In the FunOrb game Armies of Gielinor, set during the God Wars, one of Zamorak's monsters is a sea slug, although it resembles Mother Mallum more than it resembles the common sea slug.
 * In Postbag from the Hedge, Mother Mallum states "It is tiring, for this world is more populous than any other we have encountered...", implying that she may have conquered worlds other than Gielinor in the past.
 * Mother Mallum bears some resemblance to Mother Brain, a primary antagonist of the Metroid video game series. Both characters contain the title "Mother" in their name, have some sort of psychic connection with a parasitic species (sea slugs and Metroids), and have malevolent plans for their respective game universe. Whether these similarities are intentional or purely coincidental is unclear.
 * Mother Mallum's death from being crushed by a pillar has been joked about on the forums and is often used to describe either Salt in the Wound or quests that have yet to be released with low expectations. Such jokes are often used in conjunction with other memes and Internet vocabulary. (Example: Next, they're going to drop a pillar on Lucien.)
 * The idea of mind controling slugs is similar to the series "Animorphs", where there are slug like aliens called yeerks which are also able to take control of people's minds.