From the introduction:
What belongs in the Mist?
The area surrounding the city of Phlan has been dramatically different ever since the Maimed Virulence, Vorgansharax, fought back against the combined efforts of the Harpers, Zhentarim, Emerald Enclave, Order of the Gauntlet, and the Lord’s Alliance. Heavy mists crawled from the Quivering Forest and grip the land, choking out joy and happiness, and the elves have begun whispering of strange noises and a lack of woodland creatures.
An expansion for DDAL04-02 The Beast
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This supplement is designed to expand and enhance the experience surrounding DDAL04-02 The Beast. While this content does not change the XP, treasure, renown, or other rewards as presented, you may find that your understanding and appreciation of the environments presented in that adventure may be enhanced after reading this document. While this material is intended for DM use, players may enjoy reading it down as well – but be aware that it may spoil the experience of playing the adventure as well as others throughout the Curse of Strahd season!