No Man Born of a Woman

So basically, like the Witch King, the Forever King is killed not by a man but by a woman. While Gabe managed to get Ashdrinker to break through his sternum, by that time Celene had drunk the lost drop of his blood, which is why her face is fully healed because eating the soul of a vampire progenitor was enough to heal her. In the end Gabe didn't kill Fabien, Celene did because she wasn't a man