The Bioluminescent Cavern (夜花草, Yoru hana kusa, Night flowers and plants) is an area accessible from the Victorian Drains through the yellow arch with neon-looking arms.
All things in this world are glowing, slowly changing it's palette. Multicolored glowing fish swim across the screen with similarly colored rocks and marine plant life. Here you can find one of the eggs.
Nexus → Red Streetlight World → Gray Road → Grass World → Green Neon World → Construction Frame Building → Cog Maze → Forest Pier → Abandoned Factory → Arc de Pillar World → Flooded Ramparts → Mansion → Industrial Maze → Overgrown City → Victorian Drains → Bioluminescent Cavern