Exiled troops. There are a few ways to get them. End a war with them inside another nation, lose military access from a nation that they are situated in or even sell land which they were in. Exiled troops remain exiled until they land in a province controlled by you or someone you have military access from.
While exiled, the troops will not fight. Not enemies, not rebels, not natives, they are simply invincible to all but attrition. They make excellent spies to lift fog of war and can walk on
any land. I had thought that they could even walk through Terra Incognito but sadly, that does require a conquistador. None the less, I used them to "island hop" most of the way West.
So what can you do with exiled troops? After some initial testing, me and the guys who
fought against Rome along side me in the quest for Pink set out to see what could be done with them. We had hoped that the AI could be fooled by them and develop a "black curtain" tactic but the AI was having none of it. For a while these exiled troops were looking a bit useless, until we saw that they loot.
Invincible troops, and they loot.
If you are a dev and reading this, there's one for the bug list. Also, say hi!