Blood?

[Jasper]

I like the idea of varrying the effectiveness of most units blood hunting according to the controlling faction!

A similar thought is to varry the effectivess of hunters depending on the power of blood mages present. So mundane units couldn't find slaves without blood mage present to tell them where to look, and hunting efficiency would increase (but taper off) as more/more-powerfull blood mages are added.

This way nations with access to blood mages would have an advantage, without hard coding it. It would also give you some reason to recruit blood mages beyond the few needed to summon demons.