I understand that the ISWG came out after Season 0, and this seems to have caused a minor inconsistency with this scenario. In Perils, The Black Marguis claims jurisdiction over the Sellen river.
However, looking at the river kingdoms map in the ISWG, the Sellen flows well east of his Protectorate, and the river which flows through his territory appears to be the Dagger river.
Which river does the Black Marquis actually control?