It seems we've largely missed the Milthm x Rain World Collab...

For context, said collab was quite something out of the ordinary, in contrast to other prior cross-RG collabs in the past few years, with four or five songs from the environmental survival game "Rain World" - composed by James Primate and Lydia Esrig, making it to Milthim, a recent cross-platform, multi-lane RG being heavily inspired by Rizline and (to some degree) Groove Coaster), on February 23 this year.

Look further into the naming, and both games share a common weather theme - that being of rain.

In relation to that, here's MusicFracturer's first dive into said chapter.