I'm imagining that Alexi had his own batch made special from the old recipe, having Joseph get the mass produced stuff to throw him off. I'm glad you like the idea :P

Well, the result. . .For now just focus on how it will make Erik feel. Since Alexi will be supporting his will rather than repressing it, as well as Alexi basically sharing his soul with nothing in return, it would come across as a rather pleasant thing, if very confusing. I see a soul share, with the original recipe, sharing not only feelings, but memories maybe. Not the exact happenings, but those little snippets of times in Alexi's life where he felt his strongest passions. Alexi would be sharing the kindest parts of himself with Erik, you're free to come up with your own interpretation of how that feels to Erik. He's not forcing Erik to love and admire Alexi, he's simply showing Erik how he feels about the vampire and his cause. Rather than forcing his feelings to change towards Alexi, he's simply showing Erik his feelings, and letting Erik decide what to do with them.

I can see Erik not believing them at first, just like he doesn't believe Alexi's words, but by the third dose it would be clear that the feelings are not a lie. The result. . . Again, that road leads to the big plan, and you told me not to tell you :P

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As you will. Joseph is yours after all. Maybe Joseph tortured a member of Alexi's own ancestry and that's why the great great whatever decided to break Joseph in the first place, sharing all the suffering and pain from having a loved one tortured by the vampire.