Here's my take on Sedna, the great spirit of the sea and marine animals of Inuit mythology. She had many other names, in different parts of Greenland : Sanna, Nuliajuk (which is sometimes considered the name she had when she was still human), Arnakuagsak, Takannaaluk, or Sassuma Arnaa ("Mother of the depths"). As it's often the case when looking at the Native American world, there are no written records of her story from the peoples who believed in it ; it is therefore necessary to rely on the research of anthropologists to have a good idea of the legend that surrounds her, although some details change according to the sources.
According to the most common version, Sedna was originally a young human woman, who lived with her father and mother (in some versions the father is a widower). The father being a skilled hunter, the little family never lacked food or furs to keep warm. In truth, their comfort was so great that Sedna could not bring herself to take a husband and leave the family home ; she rejected every suitor, even when her parents insisted. However, one day they received a visit from another great hunter who promised the young woman to bring her the same comfort that she knew with her parents. This time, she accepted the marriage, and the man brought her to the island that would be her home. However, once there, he revealed himself to be a birdman (and/or a shaman), and only good at bringing her fish ; Sedna was first angered after being deceived, then knew unhappiness and resignation, facing her complete isolation on the frozen isle. Later, Sedna's father came to see his daughter again (either to visit her or to help her, having heard her lamentations in the distance). Realizing her misfortunes, he made the decision to bring his daughter back to the family home aboard his kayak. But the birdman caught up with them, and unleashed a storm (in another version, it was a sea spirit in his pay who took care of it). Fearing to sink in the icy waters, the father threw his daughter overboard, hoping to appease their pursuer ; but Sedna clung to the boat, and her father cut off her fingers, leaving her to sink into the ocean to die... However, her anirniq (soul), instead of returning to the Siluap Inua (a kind of immaterial fluid encompassing the universe, from which all souls come), was transfigured. From her severed fingers emerged new aquatic creatures - which she could control - and she became the great spirit of the sea.
Henceforth, when the seas raged or when marine game ran short for long periods, it was attributed to Sedna's displeasure. Her anger could be due to the prolonged absence of offerings (pieces of meat, bones, or worn tools) or to the fact that she could not untangle her long hair with her mutilated hands. The angakkuq (shamans) then had to go to sea, and symbolically "comb" her hair to appease her and get back into her good graces. Sedna's favors were indispensable, given the central importance of marine game in Inuit society ; her story encouraged men to respect women, and to be humble towards forces of nature.
In another version of the story, it is said that she was once an mistreated orphan, and that her tragic death was the doing of other humans ; yet another variant tells that she was thrown into the sea and mutilated because she had chosen to marry a dog, disappointed by her human suitors. Finally, another version makes her the daughter of Anguta, sometimes supreme god for certain Inuit tribes, sometimes psychopomp god who escorted the souls of the deceased to the Adlivun (underground/submarine and temporary sojourn of the dead) ; always according to this story, by transfiguring herself, Sedna also became the regent of this netherworld.