The Eternal Spring is a magical spring locked in a parallel dimension. Only nymphs can access it, and their satyr is the only being that can command them to open it. The magical waters grant immortality and invincibility to the drinker, and can heal any wound, no matter how severe. However, no one is allowed to drink from it. Each nymph carries a drop of the spring water to remind themselves of their responsibility to protect it.
When the demonTull drank from the spring, the Charmed Ones could not vanquish him, so they transformed him into a tree with a spell.