The Fire Spring is the eighth and final "Your Sanctuary" location in EarthBound. Its guardian is the Carbon Dog, who alters into its second form after it is near death, the Diamond Dog. After Ness learns this melody from the Fire Spring, if he has collected the other seven melodies, he is transported to Magicant and is rendered asleep until Ness' Nightmare is defeated. When Ness wakes up, the party automatically teleports to Saturn Valley to meet with Dr. Andonuts.
Interestingly, this is the only "Your Sanctuary" boss that has two forms instead of one. Fire Spring is located in the Lost Underworld. Ness has the feeling that he was being watched by himself as a baby in this sanctuary.