[AlexanderHowl]

That would be more of a feature of the undead than an aspect of the paladin magic: Healed by Necromancy spells/Harmed by Healing spells [0]. In that case, I would have undead receive 1 HP per FP spent on a H Necromancy spell and 2 HP per FP spent on a VH Necromancy spells (any Necromancy spell could target them for healing purposes) while they would suffer 1d HP per FP spent on a Healing spell (any Healing spell could target them for damaging purposes).