During the Halloween season, there was a special event called the Blood Moon Festival offering new follower forms, a new ritual and deco items for free. Technically this event is over and "unobtaineable" until next October, when Devolver Digital might repeat it.
However, you can also just go to your Switch settings and temporarily set the console date to a day between 24th Oct to 10th Nov 2022 which will make the event availiable, provided you're on the most recent update. The same tactic might work on other consoles and on PC, but personally I have only tested the Switch version so I only know that this one is accurate.
Don't forget to change the internal clock back to the original date and time after activating the event, otherwise it might affect other games on the console (like making Resetti yell at you in Animal Crossing for tampering with the clock, or removing otherwise availiable Pokémon in the Wild Area in Pokémon Sword/Shield). Going back to the normal time and date will not delete the follower forms and items you have obtained "during the event" so you won't lose anything ;)
Perfect ^^ Thank you!