The Eaten One is the elemental personification of water in the Four. She has the form of an enormous catfish, and a beautiful fishy woman.
The Four were all humans who died at the time when the Kencyr temples activated in Rathillien. The Eaten One was a maid—likely a girl from one of the hill tribes—who tried to seduce the River Snake to stop an earthquake, and was eaten. On the Spring Equinox, the Merikit act out a mummery of this event.
The Eaten One also has elements of other stories in her as well.
↑Honor's Paradox, "Chapter XII: A New Favorite", IV — "Its eyes were the size of ships’ wheels; its bristling whiskers, spars. The cavernous oval of its mouth gaped. Something like a pallid tongue flopped out to scrabble with overgrown nails on stone. The Eaten One existed only from the thighs up, the rest stuck down the catfish’s gigantic maw. Her skin glimmered pale green; her hair draped like seaweed on a low-tide shore. But the face that she turned upward was of transcendental beauty, even with its lambent eyes and needle teeth bared in a smile."
↑Seeker's Mask, "Part VIII: Kithorn" — " 'There was a maid,' it said. 'Oh, so beautiful, so proud. […] No chief's son would do for her, oh no. When the earth shook, what must she needs try but to seduce the River Snake itself. […] Poor maid,' said the voice, suddenly dolorous. 'Poor, pretty maid. The Snake ate her all-l-l-l up.' "
↑Honor's Paradox, "Chapter X: Spring Equinox" — " 'Then, too, one of your stories says that the Maid was eaten by the River Snake, another that she stuck halfway down the gullet of a giant catfish.' "