My take on Auril's Motivation for Endless Winter and Ythryn
Backstory: I'm preparing to run Rime of the Frostmaiden for the first time so I'm doing lots of homework to have it all make sense. One of the major loose ends seems to be linking Auril to Ythryn.
One of my favorite games of all time is Chrono Trigger and there are a ton of parallels between the Netherese empire and Zeal. There is also a time travel aspect in Rime, specifically in Ythryn. What caused the floating cities of Zeal to fall? Lavos caused Ythryn to fall and if crashing on the planet or expending eldritch energy caused Lavos to go into a slumber, wouldn't a mythallar's power be able to supress it? At least...for several millenia. All that chardalyn lying around makes me think that instead of demoic influence causing madness, maybe it's really Lavos's influence. He is an eldritch alien horror, after all.
Auril noticed a disturbance in her frosty abode, as if all life was being sapped from the environment. She determined the cause of this disturbance to be inside the Reghed glacier which was now actually melting! Her answer was to freeze the region, hopefully reversing the thaw and causing whatever was inside to go back to sleep.
Xardarok is still drawn to the surface with intent to collect chardalyn and build a dragon (basically a dragon engine - more Chrono Trigger parallels!). But instead of Deep Duerra being Asmodeus in disguise, it's Lavos! This ties the duergar into the other main conflicts much more. Lavos can only influence from afar, so it doesn't particularly care about building dragons or destroying Ten-Towns, but it's empowering Xardarok to slowly gain more control over him and his minions.
Auril will not be confronted in Ch. 5, I am taking an idea I saw that an agent of Auril, a frost giant druid, rides the Roc and casts the spell renewing The Endless Night. Auril herself will appear to the party earlier in the campaign, warning them of a greater threat than herself. While Auril is still an evil minor god, this could create a "lesser of two evils" scenario and make the party ally with her for a time (or not).
In Ythryn the party could still encounter the demi-lich, who was called Queen Zeal, and had tried to use Lavos's power for herself. She is the one who brought Lavos's shell to Ythryn after the fall, though doing so left her permanently drained of power. Despite her best efforts, she was unable to harness Lavos's power for herself and has been forced to endure being near its power and influence ever since. She has gone quite insane from this, though she is able to communicate telepathically to some extent. She would also gladly join a party in fighting Lavos.
The showdown with Lavos could involve Auril's assistance in fighting Lavos, who is not quite full power but still more than capable of waking up and causing a climactic fight at the end of the adventure, potentially transporting the party through time at the end of it! Lavos has a lot of power, even while slumbering. The next campaign becomes a time traveling adventure to stop Lavos from ending the world of Faerun!
If you read this post and had any furhter ideas for expanding this idea please share them.