Pandemonis
Warlord
Meant to write this up for the Sidar. But I accidentally thought of the Calabim instead
Unfortunately, the mechanic relies on the strategy and style of the Sidar, and the units and design of the Calabim. Thus making it absolutely useless for direct use in FfH. Besides, from what I hear the Calabim are already far too powerful anyhow (never used them myself as yet because of that).
Thrall
Calabim Special Unit, Replaces Worker
Starts with Promotion: Thrall
Grants Promotion: Thrall to all units in same tile
Same as normal worker for stats/abilities. Cost is 10% higher though.
Promotion: Thrall
Unit cannot attack Vampire units
Grants Promotion: Thrall to all units in same tile.
Vampires auto-convert any unit with Thrall Promotion without combat (like as if attacking a worker)
Intention was that you could gift them to your opponents and hope that they would make use of the free worker instead of deleting it. Then eventually forget to keep the other units away from him, and then BOOM, a whole civilization just waiting for you to claim them.
And even if that didn't work, at least they would think twice before attacking your defensless workers when they attempt to invade your territory. That or they would have to delete the worker and the unit who captured it immediately to prevent you from taking them over.
AI would never, ever handle this well.
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

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'"
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'"