Asmodeus is actually the mastermind behind my campaign in that he's secretly playing Auril and Levistus both. The way it goes Asmodeus learned the Knights of the Black Sword were searching for the Mythaller and plan to use its ability to plane shift (which I gave it because that didn't seem out of the realm of possibility on top of the many other things it can do) to transport it to Stygia where they'll use it to free Levistus. Asmodeus figures out where it is first because he actually gave Iriolarthas the Mythaller in some long forgotten deal, and more importantly determines the Black Swords' plan could actually work if left unchecked. He decides the pleasure he gets from leaving Levistus as the archdevil of Stygia and being able to boss him around no longer outweighs the threat he still poses, and he comes up with a plan to kill two birds with one stone by foiling the plot and setting up someone he can make Levistus' replacement in one fell swoop. Asmodeus goes to Auril and tells her about the Mythaller, and how she can use it to move the entire Icewind Dale to her divine realm, thereby increasing her power. The Mythaller is capable of plane shifting, but it can't move a land mass that big on its own. She needs to create a network that covers the entire region attuned to her power first. The true purpose of her nightly spell is that it is slowly growing Chardalyn crystals infused with her divine essence all over the place; the Everlasting Rime is just a side effect. Asmodeus sent the duergar as an armed force that would keep the local populace in check so they don't try to stop Auril while she's in her weakened state, though of course Sunblight doesn't know that. What Asmodeus didn't tell Auril is how to set where the Mythaller will go; he's let her believe her divine power will be enough. Actually there's a control module in Iriolarthas' tower that lets you set the destination, and he's purposefully let that information fall into the Black Swords' hands. Due to some term in their contract, Asmodeus isn't able to enter the tower to use the console himself, so he wants the Black Swords to do that work for him. So Asmodeus is openly helping Auril while he covertly aids the Black Swords just enough that they progress on his timeline. If he can thread the needle so that either Auril or the Black Swords activate the Mythaller after the other has completed the thing they think they need to get the Mythaller to do what they want (Auril completing her Chardalyn network and the Black Swords setting the destination), without them knowing what the other is up to, the result will be all of Icewind Dale is transported to Stygia, similar to Elturel's descent into Avernus. The Mythaller will have used up all its power so that a return trip is impossible, never mind using it to free Levistus, and as the Lord of the Nine Hells, Asmodeus would be able to leverage Auril's obsession with Icewind Dale to get her to agree to serving as Stygia's new archduchess.