History: The breaking process was originally invented by Vampires themselves, but not as a slavery tool, or a tool at all. The magic was a secret known only to a very few vampires who wanted a way to bind themselves to the ones they loved in a way no other creature could. A kind of bonding that makes love look like simple affection. When two beings loved one another so much, these vampires would create a batch of the special potion and over the course of the week, the two beings would become essentially of one heart. In other words, it was the ultimate blending of souls.
However after the upheaval the ones who invented the potion were killed and the recipe stolen by humans. At first they had not idea what they had, but after finding some old notes, a particularly sadistic scientist figured out how it works and how to use it to the human's advantage. By administering it to vampires with a human's blood, only the feelings and wills of that human would be shared, and rather than becoming one in a loving sense, the anger, hatred, and lust to dominate would break the will of the vampire and forever put that creature under their thumb.
The recipe itself remains unchanged, as well as the manner of delivery, but since one only hard breaks a slave for one reason, there has never been any other results than a being with a broken will and subservient attitude. The crushing weight of the human's loathing would overrun the non-human and create the perfect slave.
However, as I have said, in this case, Alexi has nothing but love and admiration for the rebel leader, his passion for freeing the slaves on par with Eriks. Since the potion was kept such a secret, even the ones having used it not really knowing what was happening all that well, the legends about the soul sharing potion has never been linked to, as you put it, the Resistance horror story breaking.
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Yes to what happened to Joseph. As far as the first part goes. I leave it up to you whether you want it to be linked to his self loathing. I can see it being the reason, but again, it's up to you. I can see it the other way too, as a choice made by a shamed vampire who tortured the innocent. If that's what you want, so shall it be. He's your character after all. *Smile*
So, what cha think?