Balefire is a tainted form of fire, an unholy green flame fueled by the corruption of the Wyrm.
Named after the faerie fires of Beltane, it has certain characteristics of radiation and is usually employed near nuclear sites. Black Spiral Dancers sometimes expose themselves to it in order to achieve some form of enlightenment (although they usually only gain severe burns and brain tumors).
Balefire appears to be greenish flame, yet with more consistency, almost like molten wax. It flows and bubbles and roils, glowing with its own internal incandescence. Some Pentex scientists believe it is corrupted ephemera, the "physical" substance which makes up the spirit world.