Everdark sentient pest

I was fighting everdark gnoster and there seems to be a mechanic that I am not aware of. While we were two focusing on faurtis the third member of the party came to kill gnoster, however animus never ressurected him. Why is that so?

Is it that animus cannot ressurect gnoster if it is taking posession of faurtis? I never completely understood the resurection mechanics of this fight. Does anyone know why gnoster never came to be possessed by animus despite getting killed?