Your vulnerability to an energy type is reduced.
Prerequisites: Vulnerability to energy, Iron Will.
Benefit: You can partially suppress your vulnerability to a type of element or energy. When subjected to an attack based on that type of energy, you take one-quarter again as much (+25%) damage, regardless of whether a saving throw is allowed, or if the save is a success or failure.
Normal: A creature vulnerable to an element or energy type takes half again as much (+50%) damage as normal from that energy type.
Source: Draconomicon, page 74.