[Norfleet]

Are they holding the blood slaves personally, or are you using mules in the back? Are there any "stray" mages consuming the blood slaves before your Skrattis can use them? It seems like you have a reasonable setup that SHOULD work....unless.....aha!

I see the problem.

The problem is that the mages are being too smart for their own good: They're not casting because Hellbind Heart requires gems to cast....and a lone enemy commander is, apparently, unworthy of having gems wasted on him. So your mages refuse to cast the spell.