The Swamp Fane is one of the Three Wilderness Fanes, located in Ivlis Marsh. It grants the caster several elemental resistances. If caster's connection to the Swamp Fane is severed, he loses resistance to Fire, Cold, Lightning, Thunder, Acid.
The Three Wilderness Fanes were once three sacred sites in Malphas, each guarded by one of the priestesses that are known today as the Three Hags of Malphas. When the Caster corrupted the land with dark rituals, the Fanes were corrupted. Each one gives a concrete and continual mechanical benefit to Strahd. Each priestess turned hag dwells in the fane protecting it. If the caster's Links to the fanes is cut; he loses the benefits provided by that fane.