The Leviathan is a sea monster spoken of in Gielinorian lore. There are many legends surrounding the Leviathan, although there are few concrete facts regarding it. The periscope of the penguin-controlled Red Raktuber submarine from Hunt for Red Raktuber has been mistaken for the Leviathan in the past.
Shortly after the end of the Eastern Sea Civil War, the celebrated pirate resistance leader Gentleman Mallard was apparently attacked by a massive squid-like creature off the coast of Mos Le'Harmless. Although his crew managed to escape in lifeboats, Mallard fought the creature until his death. Several days later, one of the creature's tentacles washed up on the shore of Mos Le'Harmless, with Mallard's rapier embedded in it. The creature responsible for Mallard's death may have been the Leviathan or a similar species of kraken-like animal.
In the RuneScape novel Betrayal at Falador, the character Castimir mentions that at one point he stayed in Catherby on his trip from the Fremennik Province to Taverley. Apparently the fishermen and sailors of Catherby told him what they claimed were true stories, some of which included flying fish and massive sea monsters that in some ways resembled the Leviathan.
At least one "Swamp Creature" is known to inhabit a stagnant river in southern Morytania, near the border of the Sanguinesti Region and Mort Myre Swamp. The creature is known to attack boaters travelling on the river, although the area is land-locked and thus it may be totally unrelated to other Leviathan sightings. There also exists the possibility that, when the Eastern Sea's waters receded from parts of Morytania during Lord Drakan's capture of the Hallowland, it left saltwater species trapped in inland water sources. The swamp creature may be a mutated descendent of the Leviathin species, as it bears striking similarities to creatures in Leviathin myths and its habitat may have once been covered by the Eastern Sea, where many sightings are centred.
In the Year 169 of the Fifth Age, the Dorgeshuun explorer Zanik was aboard an Entranan vessel travelling to the Asgarnian coastal settlement Port Sarim. During a storm, the ship was damaged when it struck what appeared to be a partially submerged object. Following the ships' arrival in Port Sarim, Zanik overheard a pirate that had been aboard the ship speculating that the object they struck had in fact been "the Leviathan". However, he travelled to The Rusty Anchor before Zanik could inquire further.
Evidence suggests that the object the ship struck was not the Leviathan, but the Red Raktuber submarine being used by the intelligent penguins of the "Motherland" to spy on the human nations. A subbmarine periscope can now be observed near Port Sarim and in other ports as part of a secret update released by Jagex. The update and story strongly hinted at a future update, which ultimately proved to be the Hunt for Red Raktuber, the sequel to the Cold War quest. The story can be found in the May 2009 issue of Postbag from the Hedge.