Slay Wyrmcaller Vile and use the Wyrmcaller's Horn to call down and slay Glacion. Then, return with the horn to Mage-Lieutenant Malister at Westguard Keep.
You were right to bring this to me, [player]. You must take help back north with you to Gjalerbron and slay that wyrm and its master, Wyrmcaller Vile!
Even now, as we speak here, the frost wyrm may be on its way.
However, if I'm reading these plans correctly, blowing the wyrmcaller's horn on his platform, outside in the rear left area of Gjalerbron, will call Glacion back. When the creature lands, destroy it!
And [player], please bring me the horn. We wouldn't want it falling back into the wrong hands.