Kamietsu, just letting you know, I had a 'relatively' similar idea with the +X|+X boosts for the card image, but I totally changed it and pursued my own idea with shadows.
But on with your card; I'm assuming that the upgrade is technically an 'advanced' poison, in that it doesn't need to wait a round to drop a creature's health by one. If the previous statement is correct, doesn't that mean that it's
less effective compared to the unupgraded Death card, where the poison can stack? Or are you more about ensuring that the Blood Assassin is gaining +AP each time she does her ability, rather than having her be a specialized Parasite/Bloodsucker?
Fun idea nonetheless.