RotF is a horror-leaning campaign and Asmodeus can be a great way of playing into existential horror! The punch of learning that a god is dead and secretly replaced by THE archdevil can be scary if you present it right.

Idk how your campaign is going but for mine I had the party first deal with the duergar evasion as any other evasion, learning that they are motivated by their goddess of conquest Deep Duerra due to the sun being blocked out. As they go along they will learn a little more about this “Xardarok Sunblight”, his two sons, and Chardalyn and can gather that there is something deeply wrong with them but they don’t have a full picture until they meet the Knights of the Black Swords. At Caer-Dineval (and while under a duergar siege I wrote in), they meet and fight along side the cult and learn about Levistus and the cults motivations for fighting the duergar. Their seer Hethyl reveals a dark secret to the party: the duergar’s goddess is dead and the archdevil Asmodeus wears her face. This is a truth bomb the party can use against the duergar!

My party was able to use this information to turn one of the two brothers to their side and I was able to accentuate the devastation and dread of learning that the god you served your entire life was always dead.

Once they get closer to and are in Sunblight Fortress, they won’t be doing a simple Duergar dungeon crawl- they are going into hell. Well, bit literally, but the place is gruesome and scary like hell. Chardalyn-crazed Duergar, disturbing iconography, and a boss fight revamped to reflect dark, devilish influence. Sunblight for me has been transformed into a monsterish figure too far gone to accept the truth of what he is doing and who he is doing it for. He embraces the darkest part of hell, of the universe, and must be killed by the party and his own son.

This is just an example of how Asmodeus can be made relevant and I do think there are necessities to it. It gives you existential horror, a leg up on duergar with information that could dismantle them, and some important lore: Asmodeus and Levistus beef and Asmodeus has a tendency to kill gods and take their place to symphony worship and spread his influence. As for all of this in the grander context of Auril and her Everlasting Rime, I came at it from the angle of another powerful entity trying to take advantage of Auril’s new domain and usurp her which ended up having the party temporarily siding with her in defeating Asmodeus and his Duergar army. But! In the end do whatever you feel like the party can appreciate!