Bloodstained: Curse of the Moon serves as a prequel to Bloodstained: Ritual of the Night. Just as Ritual of the Night is specifically riffing on the "IGAvania" Castlevania games like Symphony of the Night and Aria of Sorrow, which feature a heavy exploration element, Curse of the Moon hearkens back to the early NES Castlevanias (specifically Castlevania III: Dracula's Curse, which allowed the player to instantaneously switch between characters while playing) which were more linear action games, albeit with branching paths to the exit. Curse of the Moon features a much higher resolution than the NES was capable of, but its graphics and color palette were built with that system in mind.
Bloodstained: Curse of the Moon was originally developed as an early bonus to Kickstarter backers of Bloodstained: Ritual of the Night, but was later released as a standalone product. Curse of the Moon's story is set a few years before Ritual of the Night and serves to set up the latter game's antagonist and other cast members.