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.