The NPC in question: Fully Advanced Slaymate Dread Necromancer. Due to nonassociated level shenanigans, her spellcaster level is one higher than her CR, looking for CR 12 (At 13th level caster)
I'm looking for ways to tweak her out.
For plot reasons, she can have almost any sort of undead minions, but we'll keep with the standard 111 hit dice worth of skeletons and zombies that she's created, and 26 hit dice worth of better stuff that she's rebuked. She has no unreasonable sources of undead (No paragon corpse solar zombies or other foolishness, limited to the 10 hit dice for zombies and 20 hit dice for skellies)
She controls a ruined city, and is in charge of "trade", as it were. (Her master is an uttercold assault necro, and is constantly importing corpses from the northern frostfells for research, the city is a vital point.)
She has high wisdom and ranks in Knowledge:Religion, so any sort of Arcane Disciple Chicanery will be fine.
I am also considering a PrC, but I'm not sure of any that would work, although dread necro is redundant mostly for someone who is already undead herself. Also, I'm ruling her pale aura applies to spells she casts, so -1 to metamagic costs for ll necromancy spells!