Here's an idea. "Thrall_A" Promotion Req. Some Can be granted to some Calabim Unit Any unit killing a unit with this promotion will get promotion "Thrall_B". "Thrall B" Promotion Unit get : 0 if fighting unit with promotion Vampire And here's why. Spoiler : I really love this idea, planting spies to facilitate a conquer through vampirism. As TheJopa pointed, the AI would be very hard to convince to understand that, and would basically turn everything they have to the Calabim. It seems like that in MP games it would just not work either, actually: Im given a Thrall Unit. Either I make sure I never ever stack it with someone else, either I just destroy it. A Thrall Ally approach my border:Either I use HN unit to destroy them, either I just close border and not care - can be very though to handle either ways. Invisible Thralls that can enter enemy stacks... I think there might be some approaches that can be interesting. ie. A HN unit that will give a Thrall promotion to the unit who defeats it, Thrall promotion being "will always surrender to Vampires", not contaminating stacks, promotion is not visible on unit affected. (C'mon Silveric, do defeat my [enter_poor_unit]!) A HN unit that will give a Thrall promotion to the unit who defeats it, Thrall promotion being "will have n% chance to surrender to Vampires", "will have m% chance to transfer to each unit stacked with", promotion is not visible on unit affected All combination of n% and m% can be interesting. Another idea would be to compensate the potential danger by a genuine benefit, therefore excusing the visibility of the promotion and balancing its danger. I was thinking of something like get a bonus depending on numbers of thralls overall, or overall relative to those in your empire, but the design soon becomes too complicated to be good. Overall, I think the best simple design would be something along the lines of: HN unit give Thrall promotion to unit defeating them, Thrall being "Will Always Surrender to Vampire", not contaminating stack, promotion is visible on unit "You better disband your Silveric and those defenders, now, you know'"