From the necro faq in the spellcasters forum
Corpse Skin (requires target)
Target receives -15 Fire resistance.
Target receives -15 poison resistance.
Target receives +10 physical resistance.
Target receives +10 cold resistance.
Duration is ((Casters Spirit Speak - targets Magic Resist) / 25 + 40 (skill values are without decimals, meaning GM, or 100.0, is 1000)).
Mana Required: 11
Minimum Skill Required: 20.0
Reagents Required: Bat Wing and Grave Dust