Fossegrimen is a Fremennik spirit who can enchant a lyre; when played, the enchanted lyre will teleport the player to Rellekka. She can be summoned by using the offering of your choice on the strange altar to the south west of Rellekka.
Only players who have started or completed the quest The Fremennik Trials may use this teleport. Fossegrimen requires a raw fish as a tribute before she will enchant a lyre. She accepts:
a raw bass when wearing the Ring of Charos (a) (you must have enchanted the lyre once before being able to use bass), giving the lyre 20 charges;
As a part of the Fremennik Trials, players must obtain an enchanted lyre from Fossegrimen.
As the ring of Charos works on Fossegrimen, she seems to be a simpler being than she appears. According to Hamal the Chieftain, she might be of a similar nature to his daughter, Asleif, who had a piece of her spirit tied to nature upon death.[1]
After completing the easy Fremennik achievements and giving the gift for her, you will get 20 more charges - 40 for a shark/a bass, 50 for a manta and 60 for a turtle.
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Fossegrimen is based on the Fossegrim, a mythical Scandinavian spirit who lives in waterfalls and is neither good nor evil.
She cannot be talked to despite having such an option, as the player cannot reach her.
The ability to charm her into accepting a sea bass instead of a shark may be a reference to Dr. Evil, who asked for "sharks with frickin' laser beams attached to their heads" but whose researchers were only able to come up with mutated sea bass instead.
Her examine text implies the entire water body that surrounds the Fish Flingers island, only enclosed from the ocean by the two paths to the Lighthouse, is one large lake, making it the biggest lake on Gielinor.
^Hamal the Chieftain, "Mountain Daughter", RuneScape. "And perhaps you have come across Fossegrimen? ... Now that spirit is quite different from the Draugen… she is, perhaps, not entirely dissimilar to the Mountain Dottir."