From the introduction:
N5: Under Illefarn campaign book set in the Forgotten Realms, during the era described in the original Forgotten Realms boxed set (the “old gray box”). It was one of the first adventures published for the Realms, and, as such, remains a fan favorite more than 30 years after its publication.
This unofficial revision of the module is an update that attempts to stay true to the original module, while updating the game mechanics to Dungeons & Dragons (3.5 Edition), the history to reflect stories uncovered in countless other game products (including A Grand History of the Realms, City of Splendors: Waterdeep, Dragons of Faerûn, FR11 – Dwarves Deep, Lost Empires of Faerûn, Serpent Kingdoms, and Volo’s Guide to the Sword Coast, among others), and the story to fill in a backstory that ties the events more strongly into the Realms.