Barbarian spy units, they would require a base, it could be invisible and they would have a particular civ (maybe a few) that they would be aimed at. They can approach other civs for aid, the approached civ can either help them with money/espionage points/units or sell some espionage points to their enemy. You (the targeted civ) don't know about them until their first attack or if a civ (that has been asked to help them) tells you about them. Presumably they are demanding something, a city, something political, gold. When the "terrorists" strike their mission has a chance of success and they have a chance of being caught, as with normal spies. If you catch one you gain intelligence against them, when you have caught up with their intelligence against you, you can see their base but not its contents (military) so you can either drive in there with a huge army or wait until you can run espionage against them and try to find out more. Of course you could just give them what they want but this would increase the likely-hood of more.
Of course this depends on espionage working slightly different, I am not too sure I am keen on the way espionage points work at the minute, I mean its acceptable, but the rate you get points against a civ should depend on them at least a little. I mean I am sure it should be harder to gain points against Russia than Zimbabwe, or is it just a case of Russia has more so gaining 100 points against them means less than 100 points against Zimbabwe who has few, everything being relative and all.
