[HoneyBadger]

I like the idea. To further it (if only because I enjoy discussing the game), I'd suggest that pooling bloodslaves outside of an area equipped with both a lab and a fortress (you need a proper dungeon to lock up those slaves, and a magical lab to keep them pure), should make them vulnerable.

If a province where you've pooled your slaves should be attacked, those bloodslaves could then appear on the battlefield. That would significantly nerf the ability, while making for additional strategic choices. Scouts (only) might also have the special ability to free bloodslaves in such vulnerable provinces, while hidden, which would give scouts a nice boost of useability, without breaking their balance.