Decaying specifically says "Sets Decay, so the target will take physical damage from healing spells and potions." However, what actually happens is that anything that makes your hit points go up does damage instead, which is not at all clear and seems incorrect. So necromantic life steal damages you, blood sucker damages you, poison damages you, healing damages you. It should only effect "healing" spells, it should not effect poison or necromantic life-steal, which means undead should largely be uneffected.
Furthermore, the only reference I can find to a "healing" type damage is on healing potions and the necromancy skill. Restoration, Rallying Cry, First Aid, and Blood Sucker all "restore vitality." However, contrary to that, Restoration, Rallying Cry and First Aid damage undead while Blood sucker heals the undead, and despite the fact that Necromancy "heals" you for a % of damage dealt, it actually still heals you as undead.
What the description SHOULD be is that anything that does healing to living and damage to undead should read "heals for X," anything that heals you regardless of whether you are alive or undead should read "restores vitality for X," and anything that damages the living and heals the undead should read "poison for x."
It takes way too much trial and error trying to figure out what actually works and what doesn't and the in game text does incredibly little to clarify that.
