So, my players have finished the Trials and found the Codicil, and are perparing to depart Grimskalle - they basically don't see Auril as being a threat they can face, and based upon the information they have so far, it's hard to blame them.
I plan to change that by having the Codicil contain details about the Endless Rime ritual that Auril is currently casting each day, details that Velynne and Professor Skant (or the PCs with a decent Arcana check) can interpret, giving them an idea of just how much the ritual is weakening Auril so that they can start to consider her as something they can take on.
But I'm wondering about motivating them for the rest of the adventure. Velynne basically got them onboard for the Ythryn expedition with the lure of potential knowledge or items to help end the Endless Rime, and they see that as their main reason for going there, so once the threat of Auril is over it seems unlikely that they'll feel particularly inclined to do so. Certainly they haven't warmed to Velynne as a character and don't trust her an inch, so they won't do it on her account.
What I really need is a hook to tie in this last part of the adventure to the overall storyline. I've been considering making the Endless Rime more than just effectively a weather effect - making it more like a Domain of Dread, with Auril's spell gradually turning Icewind Dale into a separate reality, cut off from Faerun as a whole, one in which there is no sun. Even after defeating Auril, they can't actually reverse the effect without something from Ythryn.
But I'm struggling with really fleshing out that concept, and finding a way to introduce it to the players that doesn't feel forced.
Maybe I should just let them leave the island, and leave the confrontation with Auril until the next chapter, but it just feels like it'd be a bit of a footnote if she simply randomly shows up in the city.