The Rainy Castle appears in Chapters 1 and 2 of the Void Quest within the Midnight Channel on rainy days. It will alternate between casting Agilao and a Mind Charged Agilao. After being defeated, it may drop a Curious Boulder. Selling the Curious Boulder at Daidara will unlock the Amami Legs weapon for Chie Satonaka, which has the highest Attack rating for her Void Quest weapons and Counter effect, but a reduced Hit stat.
In Golden, Rainy Castle appears in Chapters 1, 2, 8 and 9. It may also drop Howlite, Milky Quartz and Bloodstone for trade at Shiroku Pub.