The only working portals are the ones to the observatory and to the hall of heros, and no dialogue or items get me out. Just singularly killed Sallow Man with my Ranger and CC'd/moved around strategically with my other characters while Lucian tore through Braccus. As Fane, you are also given two additional choices, in which your ancient mistake may be put right.
This is a relatively rare issue, which seems to be much rarer than the previous version, but it has still been reported a couple times (there were fixes for this in the last update, but apparently not covering every possible cause). I feel like the devs just threw it in as the obligatory rpg dragon fight since there is only one other in the game and its optional. Congratulations on finishing Divinity: Original Sin II! The result is one of my crew betrays (That's Ifan, I don't know why, maybe he's the weakest, or he has the least attitude toward me), the final fight is fighting with him, and he get killed easily, game ends.