Syöjätär appears in some of the "Magic Songs" (spells) catalogued by Lönnrot 1880 in the Suomen kansan muinaisia loitsurunoja - these were later translated into English by Abercromby.
[1] A possible origin story is given in a song describing the "Origin of Injuries caused by Spells", which contains some post-Christian elements: Louhiatar wife of Pohja becomes pregnant whilst sleeping with her back to the wind, impregnated by a blast of wind... After more than nine months the woman seeks to give birth but can find no good place to do so - then god (Ukko) speaks to her from a cloud indicating that a "three cornered shed is on the swamp, on the shore facing the sea in gloomy Pohjola [...] go thither to be confined, to lighten thy womb..." - she gives birth to nine sons, and one girl.
God (the Christian "Maker") refuses to baptise them, as does "Juhannes, the holy knight" (John the Baptist) - Louhitar then baptises them herself, giving them names (they become disease principles) - one of her boys, who lacks a mouth or eyes remains unnamed, and she sends him away to the Rutja rapids, from him were said to originate sharp frosts, sorcerers and wizards, jealous persons, and the creatures called Syöjätärs.
[8] In a song for the purpose "To Still Violence" Syöjätär is referenced as the "Ogress", and is given as an element in the consumption of persons consumed by violence or anger: [..] there is the Ogress (Syöjätär) in the sea with a mouth in the middle of her head, a tongue in the middle of her throat, who has eaten a hundred men, destroyed a thousand full-grown men; may she now also eat thee up, as the bread she eats, as the feast she holds.
[10] Together with ajatar (huntress) and akka (old woman), Syöjätär fills similar roles in Finnish folklore as does Baba Yaga in Russian lore.