From the introduction:
No campaign is easy for a Dungeon Master to run but the Rime of the Frostmaiden Companion ("Companion" for short) is devoted to making it easier and more satisfying for you to run. By providing new hooks, variants, flavor text, as well as any referential information for creature statistics or magic items, this supplement should make for a faithful companion to keep at your side.
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The Companion is quite formulaic; every chapter is designed to be easily digestible, whether you're a veteran reader or just diving into it mid-campaign.
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Chapter 3 is the campaign's point of no return. Once it begins, the adventurers are set on a collision course with the chardalyn dragon, Auril's island, and the city of Ythryn. Make sure you have done everything you wanted from Chapters 1 and 2 before the adventurers truly embark on Chapter 3's events.