Undine’s Spring (ウンディーネの泉, Undīne no Izumi), also known as Taboo Spring (禁忌の泉, Kinki no Izumi), is a location in Monster Girl Quest!. It’s the domain of slimes, the home of the Water Spirit Undine and the birthplace of the Heavenly Knight Erubetie.
Despite being called a spring, it stretches for kilometres and it’s 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’s found “invading their home”. Erubetie proceeds to attack Luka and almost kill him, only for Undine to come to his rescue. Erubetie decides to leave and let Undine deal with the human. Undine tests Luka by requesting that he use Sylph and Gnome at the same time. When Luka eventually succeeds, she grants him her power.
Later during the invasion, Luka and Alice visit this place to have the hero reforge a contract with Salamander, and they're surprised to see that the misanthropic slime welcomes human refugees. However, one of the monster, Slime Bess, poisons the spring with magic mercury under Black Alice's orders to be given the lead of the race afterward. Just as Luka defeat her, Amphisbaena, one of the Next Dolls created by Promestein, attempts to kill Erubetie who's saved at the last second by the hero. As the Next Doll is specifically designed to destroy Erubetie's slime through vibration, the latter is forced on the defensive, and when Luka attempts to step in, Throne Eggiel descends from Heaven to attack the hero. Once the angel is dealt with, a passably miffed Amphisbaena falls back as she can't afford to take damages.
Afterward, Erubetie is still in shock to have been saved by one of the human she despised so much while a member of her own kin nearly got everyone exterminated. She resolves herself to be more open-minded and to communicate with humans in the future thanks to a small speech from Luka. The hero then reforge his pact with a weakened Salamander, while an annoyed Alice evaporates part of the spring.