Undine’s Spring (ウンディーネの泉), also known as Taboo Spring (禁忌の泉 Kinki no izumi), is the place where the slimes have made their domain, as well as acts as the home of the third spirit, Undine, and the birthplace of the Heavenly Knight Erubetie.
Despite being called a spring, it stretches for miles and it is one of the few unpolluted areas in Ilias’s world, also being free from human settlement. Due to these factors, slimes congregate here as low-grade slime monsters can only live in unpolluted places with a pure water source.
On the surface, the area is a large serene lake surrounded by plant life, however, setting foot into the lake warps one to a linear subterranean cave system which is where the slime population live.
Through the cave, Luka fights a group of slimes, starting with Jelly Girl, then Blob Girl, then the Green Slime, the Blue Slime, the Red Slime, and the Purple Slime, only to be met by Erubetie after he is found “invading their home”. Erubetie proceeds to attack Luka and almost kill him, only to be intervened by Undine. After Undine tests Luka, she joins Luka’s party as the third spirit in his inventory.