I have read a lot about her in the past months. I did a version of her tale at MythOff USA (see below) for Slavic mythology - in that version, she was the goddess of the Underworld, who, very appropriately, escaped with Koschei, the god of the underworld.
In folktales, however, Maria Morevna turned into a princess. The first time we see her in one of the tales is across a battlefield full of dead soldiers - when Ivan tsarevits, the hero, inquires about the massacre, he is told that Maria Morevna, a princess, killed all those men.
Maria Morevna is clearly a warrior. I especially enjoy the fact that this story is the opposite of the Bluebeard tales: the prince is left home while the princess goes to war, and opens the one room he was not supposed to open, which leads to a whole world of trouble.
The princess' warlike nature is never quite explained in the stories. She just happens to be a warrior that can slay an entire army, and still look attractive when a prince visits her tent after.
This tale was recently turned into an amazing mythic fiction book called Deathless by Cathryn M. Valente. Check it out, it is a gorgeous book!