Just to throw a new nasty idea into the mix, as I’ve not seen it elsewhere…
Given that she’s not entirely dead yet, do the Legion have a very significant incentive to unearth and deal with Oysingra before the Cinder King’s forces reach her?
Yes, he’s offstage at the moment… but if she’s still alive, presumably he could Break her if she’s captured?
Worst case, an undead Oysingra would be brought back to full power, comparable to Render and Zora.
Even without that sort of boost, she’d be a formidable threat (Lieutenant level), wouldn’t she?
In my campaign (currently at Fort Calisco) I have tought my players to the plot consequences from unplayed missions far more than the -1 Supply penalty a mission might have. When explaining the mission and stating that Osyingras body must be destroyed lest the Cinder King gets his hands on it, the Commander was sufficiently freaked out that it was the mission we played.
My ingame consequences would be to add a new lieutenant to who ever of the Brokens forces the Cinder King were favouring at the moment. In my game Osyingra was all about whispers from the darkness, incorporeal form and her past history with the Last Emperor and Zora. So kind of like a Ringwraith from LoTR.
I have found that the more you emphatize the story consequences players will take it into consideration and give you new ideas as well when prioritising the missions. “Oh, I bet that if we dont do Mission A this will happen to the Legion…” I had not thought of that, but it is awesome so now it is cannon!