Shivering:The Great Divide

Quick Walkthrough
# Speak with any citizen of Split.
# Speak with either the Manic or the Demented Horkvir Bear-Arm.
# Kill all the Manics or all the Demented.
# Return to the surviving Horkvir Bear-Arm for your reward.

Detailed Walkthrough
Double Vision
Go to the town of Split just below the northernmost lake just below Overlook Road. The town is situated directly on the Mania/Dementia border, and has two copies of each person - one Manic, and one Demented. Talk with the locals, and you'll find that both sides want their opposites dead. They will each direct you to their side of Horkvir Bear-Arm for instructions, either the Manic or Demented version.

Since there are two Horkvirs, you can speak with either one. Do so, and they will both provide you with some backstory: a mage believed that every person had both a Manic and Demented side, and decided to experiment by casting a spell that split the two personalities. The two sides have since been constantly at odds, and both are vying for control of the town. Due to the laws of the realm, however, nobody is willing to get rid of the other side themselves since killing one's self would result in spending an eternity on the Hill of Suicides. That's where you come in. The Manic Horkvir will then ask you to kill all the Demented, and the Demented Horkvir will ask you to kill all the Manics. Eventually, though, you'll need to pick a side - talk to the appropriate Horkvir and tell him that you'll help (once you do this, the side you didn't choose will no longer talk to you).

Cutting Excess
Do not kill anybody of either version before you agree with one of the two Horkvirs or the quest will be failed. Travel around town and kill either all the Manics or all the Demented, depending on which Horkvir you agreed to help. Be careful though, for if you kill someone's wrong version at this point, you'll forfeit your reward and fail the quest. Make sure the quest arrow is pointing at your target before you kill them; sneaking into their houses at night will also make their alignment clear. Remember that the people from Mania usually wear brighter and more vividly colored clothing than the people from Dementia.

You need to kill the following in any order:
Atrabhi (Manic, Demented)
Horkvir Bear-Arm (Manic, Demented)
J'zidzo (Manic, Demented)
Jastira Nanus (Manic, Demented)
Urul gro-Agamph (Manic, Demented)

The killings count as murder if you're seen, so be careful. The safest bet is to sneak into the houses at night and either pickpocket a Poisoned Apple into the person's inventory or simply sneak attack them while they sleep. However, if using poisoned apples, be aware that both versions of J'zidzo have a habit of picking the pockets of the other NPCs. It is possible to fail the quest because the version on your chosen side eats an apple intended for somebody else. Don't forget that you can always just cast frenzy to avoid a murder count. However you kill one of the sides, return to the surviving Horkvir for a leveled gold reward.
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Notes
If you've already done the quest Ushnar's Terror, and the skinned hound you got as a reward sees you, you will get a bounty.
Occasionally the game will glitch and give you a murder count even when you aren't seen. It is advisable to save before each attack and check your character sheet afterwards.

Bugs
 
Killing her may require a single hit (i.e.,  a backstab), or more subtle methods such as a Poisoned Apple or Chokeberry.
Another method is to break into her house, wake her up, and stand there until she attacks you for trespassing. You can now kill her in the brawl and you won't receive a bounty.
 
 
 When all dead residents of Split are either Demented or Manic, you can use the console command   to start the quest and then immediately must speak with the "right version" of Horkvir Bear-Arm (i.e., Manic when anybody of Demented residents had been killed before starting the quest or vice-versa). In this case all dead residents will be counted as your victims.
 

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