Ranakdevi is a loa of a sati. People who hold the title of 'sati' are widows who threw themselves into the flames of their husband's funeral fire. Given her tragic death, Ranakdevi is vengeful, and uses fire as her main source of attack.
In New Orleans, Ranakdevi possesses Roland, bent on killing him and Sadie. It isn't until Sadie enlists the help of Felicie Mirepoix and Sobo Kessou (another loa) that Ranakdevi was able to be sealed into her original state, though she is still within Roland.