If your Hierophant gets his second call to heaven (or maybe hell) then it's the beginning of the end as your army literally begins to crumble to dust. All you can do to try and slow the descent is make sure your units are large and lead by a character with decent leadership.
Once your Hierophant (probably a Liche) resembles a pile of cigarette ash then your units all start to take leadership tests and lose the number of wounds they fail it by each turn. With a basic skeleton leadership of 3 or 5 for horsemen this is going to cause you a lot of lost souls in a very short space of time !
You have been warned, protect your Hierophant, he is the daddy of your army afterall !