Hello,
For an NPC and mostly bad guy, I had some inspiration from Heroe of might and magic 4: the hero of the death magic campaign is Gauldoth half-dead, who has a weird condition: in a big fire in a library in which he nearly died, he desperately did a spell from a scroll way beyond his level, which left him with his left side a living human and his right side undead.
Now, in pathfinder, Nethys has a sort of split personality down the middle as well, so i decides to make an interesting NPC which is half-undead, and has links to the church of Nethys. I will probably use the lich template for this.
This gives a lot of headaches, first of which is the reaction to positive/negative energy. This could be solved by making the NPC a cleric with the death domain, which has the 8th level power that you heal from negative energy as well as positive. (Death domain isn't one of Nethys domains, but since the NPC will be a powerful necromancer, Nethys could grant it to him as an exception)
But there are many problems with DR, natural armor, and undead traits like no constitution, immunity for mind-effecting stuff, etc.
Does anyone have ideas or this?
Pathfinder has no system for attacks hitting the right or left side. I could use a 50/50 chance to decide which side an attack hits, but that seems rather unfulfilling, so i need help there too.
Just make him as 2 creatures that function as one.
Roll up an ECL X wizard/lich.
Divide his total HP by 2.
Half the HP goes to his "living" side and half the hp goes to his dead side.
Living and dead side can be targeted separately, exactly is if they were 2 separate creatures that shared the same space. Attacks made against the living side will be for all intents and purposes "normal" . Attacks against his dead side will have to deal with undead immunities, etc. One side will heal positive and be harmed by negative, the other will heal negative and be harmed by positive.
Keep in mind each side may have different ACs or saves.
Creature doesn't die unless both sides are defeated.
Alternately, you could have the Lich's Phylactery literally be his human side. He won't be fully destroyed unless the human half of him is dead, dead, dead.