I'm really loving this idea, and with the right requisites you can make it as flavorful as you want:
Create Undead Feat:
Requires the ability to channel negative energy or Spell Focus [Necromancy]
Allows the creation of undead creatures. See each creature entry for details on its creation.
Skeleton / Zombie:
Requires grave dirt, an onyx gem, and a body with/without flesh. (+ appropriate spells)
Incorporeal Undead:
Requires unholy water, a black gem worth X gold/HD, and a victim to be sacrificed during the creation of the undead creature. (+ appropriate spells)
Morgh:
Requires the body of a mass murderer (must have killed 10+ defenseless intelligent creatures for pleasure or selfish goals), ashes from one of his victims, and an obsidian shard. (+ appropriate spells)