The Black Flag is a part of Tyvian Lords.
This is the rulership position for the realm.
A plunderer for hire.
If you see this flag over your estate, then I or one of my underlings has stolen it. How distressing for you! What is worse, by the time you come to reclaim it, we will be long gone, back out into the rolling waves of the sea, and poised to strike again!
All is not lost, though, because I may not be your true enemy. Someone may have hired us to do this! Of course, I can't reveal my clients, but if you want to strike back in anonymity, use the stone left at your estate to send me or my vassal a missive.