Lockshackle. Can't use enemies in prison
Map Lockshackle from My pet dungeon.
When I captured 2 prisons from the empire, inside I discovered several imperial units. I can not take them in hand, I can not use them for blood money. But I can hit them with lightning.
This has happened to me on a few occasions as well. The empire creatures appear to be physically inside the prison, unable to leave or attack my minions, but not "imprisoned" (so I can't pick them up, Blood Money them, and so on). In order to resolve it, I generally have to sell prison tiles in order to place them outside the bars, at which point they start behaving like a proper Empire unit again.
I'll see if I can come up with a reliable repro case.