Well, I'm biased of course being the developer/IP owner of the Kaidan setting of Japanese Horror, which notably is for PF1, not D&D per se, but The Curse of the Golden Spear trilogy of modules (5th, 6th and 7th level), and perhaps the first module: The Gift is a grand start, though playing through all 3 adventures is required to receive the full introduction to the setting. It was designed as an intro into that setting from the beginning. Because the adventurers are not locals, rather "Westerners" as bodyguards to protect the goods of a merchant visiting Kaidan for his second trip, so as non-locals there are no preconceptions on the expectations of adventuring in Kaidan. It's all new to the adventurers, just as it is intended to be new and beyond expectations of the players. The social caste system and how it impacts members of various castes - learning each's rights and restrictions dwelling within a police state (that is what feudal Japan was) - this is introduced in the first module, even in the first chapter of activities there. The document heavy requirements needed by adventurers to even travel across the country is introduced. The various kinds of undead, monsters and unique hazards that residents of Kaidan face are introduced. How the undead, ghosts, impact the lives and messed up, reincarnation cycle of those in Kaidan are introduced. Every niche aspect of Kaidan is introduced across the 3 modules. To me, it's the best intro adventures in existence, but as stated I'm very biased...