This is the reverse of the Power: Balefire Flux. Whenever the fomor is exposed to Balefire, the toxic excretions of the Wyrm, he may undergo a horrible transformation, gaining a new Taint.
After the fomor undergoes five turns of successive exposure to Balefire, roll her Stamina (difficulty 9). At the Storyteller's option, particularly extreme exposure to Balefire may lower the difficulty of the Stamina roll. If successful, the fomor gains a new Taint, chosen randomly. This new "curse" from the Wyrm is instant--the character immediately undergoes a painful transformation for three turns, during which she can take no actions, not even to defend herself. Afterwards, her new Taint will display itself (such as an arm falling off, if she is inflicted with Rotting).
This Taint does not make the fomor immune to Balefire; unless she purchases Immunity, she will suffer any normal damage from the toxin.