The Heartspring is a sacred shrine dedicated to the Great Shalm, Obad-Hai. It lies deep in the Tangled Fens northeast of the Turimori border town of Fenwatch. Much of the water that feeds the Fens spills forth from the Heartspring, which is why it was once revered by the druids and other natural guardians of the Fens.
Unfortunately, some time during the past year, the waters that flow from the spring became tainted with an unknown evil. This evil began to manifest in the Fens as a spreading taint of corruption and depravity. Eventually, the once good guardians of the sylvan shrine surrendered to the corrupting power of the unknown evil, descending into madness and undeath.
When our heroes first entered the Heartspring, they found its most powerful guardian, a once noble treant, standing among the scattered remains of what were presumably members of the druid circle that tended the swamps. The treant had become physically possessed by a being of elemental evil leaving the group no course but to battle him to the death.