I've had a campaign going for a year or more now and one of my players daughter of Belial, and sister to Fierna. Recently they found he has become semi mortal because reasons. She is looking to take him down etc...  Like every content release, wizards isn't going to give us much at all before dropping the book on us. I'd like to tie in that storyline so I can get my party there via baulder's gate when the book comes out. However I'd like to point them in that direction now because it will take some time to wrap up their current arc and move in that direction. Any good ideas to hook that direction, teasing the war in avernus while leaving things open enough to tie in once the book is actually out? Ideally they would have to play through alot of the avernus campaign to wrap up that arc.

We have 3 lvl9s, a lvl 11, and a lvl 13

Perhaps something similar to Out of the Abyss, where while the party is adventuring, they stumble across some devils and demons, or celestials and fiends, fighting each other.