Shax, the Sea Sister, was once goddess of the sea in the Giant Pantheon. A daughter of Annam, she had dominion over storm giants and the waters. She was a cruel goddess, inspiring the storm giants to battle other species. For some reason, Annam could not sense her, and never knew she existed, perhaps in the same way that he didn't know of his daughters Diancastra and Hiatea until they introduced themselves to him.
Thrym found his sister beautiful and terrible, and offered to wed her. She refused, and so he decapitated her, thus saving the storm giants from eons more cruelty. The fragments of flesh that fell from Thrym's wounds became the first icebergs.
Source: Tome of Magic (D&D 3.5)