The Frosted Prince

Publisher's blurb: Just over a century ago a great battle took place at Castle Krag. The place served as a Zhentarim stronghold, but was cleared out by adventurers. Now, strange things are occurring once again in the ruined castle. For miles around, the weather is unnaturally cold, and shrouded in an impenetrable mist. What could be causing these strange effects? A 6-10 hour adventure for 11th-16th level characters