Fossegrimen is a Fremennik spirit who enchants player's Lyres so that playing the Lyre teleports the player to Rellekka. Only players who have started or completed the Fremennik Trials quest may use the lyre teleport. Fossegrimen requires a raw shark as payment for enchanting a Lyre. If the player equips a Ring of Charos (a) then a raw bass can be substituted for a shark. Offering a raw shark will grant 2 charges. Giving a Raw manta ray to Fossergrimen grants the lyre 3 charges, while a Raw sea turtle can be used to obtain 4 charges. The Fremennik Trials requires that players must obtain an Enchanted Lyre from Fossegrimen.
The picture shows the altar the player must place the food in to summon Fossegrimen and have the lyre enchanted.
Also where on the map the altar can be located at.
Trivia: Fossegrimen is actually based on a Norwegian mythical creature that shares her name.