Paraphrased from Wikipedia:

The Baba Yaga is a supernatural being from Slavic folklore. The Baba Yaga appears as a deformed and ferocious-looking witch and flies around in a mortar, wielding a pestle. The creature dwells deep in the forest in a hut usually described as standing on chicken legs (can't make this up). The Baba Yaga may help or hinder those that encounter or seek her out. The Baba Yaga can be considered the "dark" to Bahamut's "light".

The common phrase: "May the Baba Yaga not take you this night" relates to this being, as the Baba Yaga is known to steal people away in the night. You may however want to taken if it's Merry Berry Day as she'll give you berries.