I've tried to imagine how it would work. The best thing I could come up with is similar to Omega Red in Marvel Comics. He has a "death factor", to oppose Wolverine's healing factor. How it works is his body creates a 'death spore'. He must transfer this spore into other people or the spore feeds on him. The more spores he infects people with, the better and stronger he gets.
I can see the BC having access to a source of power. The amount of energy they can draw depends on how many 'batteries' they can put the power into. They kill someone, then use them as a battery. The more batteries, the more power.
Or they establish within the corpses a connection to the power source. The more corpses, the more portals, the more power they can draw on. The longer the corpse is in their thrall, the better attuned it becomes to the power and the more it can channel. Sort of like the rules for necromancers and zombies.