...I really don't know how or when this happened, but somewhere along the way in my main Birthright playthrough, I've managed to procure a Dark Crown. I noticed I had it shortly after having my avatar beat Leo to clear chapter 18 (meaning I'm parked in the chapter 19 position), as I was shuffling some accessories to put Orochi back in the prison and Izana in my armory (I'd moved Orochi to the armory after opting to go Basara with Hayato to keep tomes on sale; Saizo manned the prison in the interim), but I have no clue how I managed to actually get it. Now that I've gotten it, its full descrption is now available on the wiki. I'm going ahead and sharing a few of my Birthright file's statistics as of my last save (with only one further battle after having recruited Izana) in case they might play in to how I managed to acquire the Dark Crown and could serve as a key to unlocking the mystery as to how I managed to obtain it:
Accessories: 1858
Bond units: 149
Battle points: 12706
Visit points: 48513
If you want to see my Dark Crown in action, my current castle update has my avatar wearing it. The address is 05002-65962-89842-33117. Note that the update where I'm wearing the Dark Crown won't be up for long--next time I hit "Update data" on one of my castles, there won't be a Dark Crown on my avatar...