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Ghost type
The Gym tasks you with looking for stuff that seem out of place, like a misplaced book in a bookshelf; interact with it and it turns out to be haunted, and you can actually catch a wild Ghost type Pokemon that can possibly help you in the Gym, but you also have to be careful since Ghost is also weak to itself
Placing stuff where they actually belong will open the way forward
First team would consist of Frillish, Lampent, Trevenant, and Dusclops
Rematch team would consist of Froslass, Jellicent, Chandelure, Trevenant, Runerigus, and Dusknoir