The electromagnetic adaptive camouflage system
The biggest reason that Amerikka Command survives to plague the Atlantic Federation, the only reason the ACPS hasn't destroyed the Python Pirate Syndicate, is their mastery and sharing of shroud technology.
A group that wished to be 'ever-living' instead was cursed with 'never-dying'. Thier flesh rotted and fell fromthier bones but still they lived on. Now as skeletons they continue thier quest to remove thier curse. As skeletons they differ in that they do not need controlled or summoned. They are fully fledged NPCs with drives or ideas of thier own. Stabbing and slashing weapons would not affect them.