The Black Fire is a mysterious bright green slime, in which the Dark Sorceress used to poison the animals of Aldenmor. It rained from the sky, dropping into rivers, the ocean, onto the land etc. It killed many animals.
The Ravenswood refugees often fell through the Ravenswood portal covered in Black Fire. An example is Lyra, who when was first found by Adriane and taken to Emily's mother's animal hospital (the non-magic users thought that it was toxic waste).