Okay, not a demon, but she absolutely belongs in this blog.
Ancient Slavic (From Russia and Eastern Europe) people would invoke a powerful goddess in a ceremony each August, chanting Mati-Syra-Zemlia, which means ‘Moist Mother Earth.’ They would recite the phrase to the compass points, asking the goddess to intervene on their behalf with the bad-weather gods, and ask the good-weather gods to visit the crops.
But the Slavs didn’t leave written records of their religious beliefs, so it’s hard to know if Mati-Syra-Zemlia was the name of the Earth goddess, or just the way to invoke her in this particular ritual. She doesn’t have any particular form, she was just.. the Earth.
What we do know about her was that you could talk to her by digging a hole in the ground and asking a question. Then you listened for the answer by listening to the hole.
She oversaw oaths, to keep people honest. After all, you can’t lie to Mother Earth.
You could also invoke her in times of plague by digging a furrow around your house, to unleash her spirit to combat the plague spirits.
She was a very powerful goddess in her day, one of the primary deities of most of the population–and you have to wonder if maybe her worship has survived in the form of respect for “Mother Russia.”
Sources
http://www.godchecker.com/pantheon/slavic_baltic-mythology.php?deity=MATI-SYRA-ZEMLYA
Mike Dixon-Kenned, Encyclopedia of Russian & Slavic myth and legend, ABC-CLIO, 1998
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