Dark Water
From Equestripedia, the Archives of Equestria!
Dark Water is a corruptive liquid that makes up the hot springs near Ponyville in the Ponies of Dark Water story arc.
History
The Mane Six were infected with dark magic when they relaxed in dark water. Zecora and the Cutie Mark Crusaders cured them using ground dragon scaled and bellwether dust mixed in water.[1]