Blood on the Ice

Background
This is a multi-step quest which begins in Windhelm. It entails discovering the identity of a serial killer that has killed at least 3 young women by the time Windhelm is entered by the Dragonborn.

The body of the latest victim is in the cemetery, and the quest is activated when the nearby Windhelm guard is queried. There are alternative paths through the quest, so following the quest markers exactly may not be optimum.

There isn't really any forensic reasoning involved; common sense will suffice for the initial portion. The warning about alternate routes through the quest should alert the player to question some early conclusions the various dialogue options seem to force. The walkthough below gives the details.

The biggest "reward" from this quest is related to the Strange Amulet. If you're going to go through all the hassle, make sure not to bypass that section if you like more magicka available to you.

Initiating the quest
To initiate this quest, the Dragonborn must travel to Windhelm at around midnight. If the Dark Brotherhood questline has been started or completed, it could hinder the possibilities of starting this quest.If this happens and murder scene doesn't appear, killing Grelod the Kind will cure this issue. At this point, the player can complete this quest in one of two ways:
 * 1) In the graveyard, there will be several people gathered around a female corpse. The guard will approach the Dragonborn and start a conversation that leads to the initiation of the quest. To get to the graveyard, one should turn left at the entrance to the city, go through the archway, make the first right and go down the steps behind the Blacksmith.
 * 2) Buying the house Hjerim in Windhelm will also start this quest. In this case continue at step 8.
 * 3) Another way to start the quest is to steal the key to Hjerim from Tova Shatter-Shield and enter the house. When one of the fliers located on a small dresser is taken, the quest will start at step 9.
 * 4) After questioning the three nearby witnesses, the Dragonborn should talk to the guard again.
 * 5) In order to be granted permission to help the investigation, the Dragonborn must visit Jorleif, and then return to the guard.
 * 6) Inside the Hall of the Dead, the Dragonborn should speak with Helgird.
 * 7) If the Key to Hjerim (purchased or pickpocketed) is in the possession of the player, the house can be entered to skip to step 9; otherwise, following the trail of blood in the crime scene will lead to Hjerim.
 * 8) Advice should be sought by speaking to Jorleif.
 * 9) Entering the house can be done by picking the lock or receiving a key from the Tova Shatter-Shield, Friga Shatter-Shield's mother.
 * 10) All of the furniture that can be investigated should be.
 * 11) The Butcher's Journal #1 from the bloody chest up against the wall on the left of the entrance should be taken. The wardrobe on the right in the northwest room of the house will have a hidden door leading to an altar.
 * 12) From the altar, Butcher Journal #2 can be found.
 * 13) The Strange Amulet and pamphlets from the small dresser just next to the secret door should also be acquired. The amulet is hidden under the pamphlets.
 * 14) Jorleif will provide the player with information, suggesting to talk to Calixto Corrium (once you have the amulet) and Viola Giordano.
 * 15) The Strange Amulet can be shown, and eventually sold, to Calixto Corrium. He will inform the Dragonborn that it belongs to the court mage. The amulet is needed in order to accuse the court wizard, although it is recommended to sell it or it will remain in the inventory indefinitely after the quest is completed.
 * 16) Viola Giordano is often found in Candlehearth Hall. She can't be talked to unless a copy of the "Beware the Butcher" pamphlet is in possession, or if she's dead or missing. If the journal has been picked up before talking to her in the house, it can be dropped and picked back up in front of her to continue the accusation quest line.

Accusing the mage

 * 1) Returning to Jorleif will end the quest, accusing the court mage of the murders.
 * 2) Waiting three days outside of Windhelm will make a body appear outside Candlehearth Hall, reopening the case and start the second part the quest, "Catch the Killer". If you are maximum level waiting 3 days will NOT work, instead, you will have to visit the court mage in the barracks jail (see bugs).

Catching the killer

 * 1) Rather than going to Jorleif, you can choose to talk to  Wuunferth the Unliving, the court mage, directly. Accusing him of practicing necromancy will cause him to deny the accusation, telling the Dragonborn that the amulet and journals are not his. He will then direct the player to where the killer will strike next.
 * 2) As soon as the marketplace area is visited (right by the White Phial shop; the map indicator will be a little misleading) in the stone quarter after dark, as Wuunferth suggests, Calixto will be seen about to murder Arivanya (alternatively, if the Dragonborn has married a woman in Windhelm, Calixto will attempt to murder her). He can be stopped.
 * 3) Alternatively, instead of stopping Calixto in the marketplace, if he is witnessed successfully carrying out this murder, he will run off into the night. The quest tracker will then lead back to Hjerim, where the killer lies in wait and will attack immediately upon entering. If the player is fast enough, they can catch up to Calixto and strike him down, with no added bounty.
 * 4) Returning to Jorleif and telling him that the killer has been caught completes the quest.

If the amulet has been sold to Calixto earlier, it can be looted back after killing him. It turns out to be the fabled Necromancer's Amulet. It grants +50 Magicka, Fortify Conjuration 25%, and reduces Health and Stamina regeneration by 75%.

Trivia

 * If the Dragonborn is part of the Dark Brotherhood, and completes the quest 'Mourning Never Comes', killing Nilsine, Tova will be dead on the floor next to a suicide note in her house. Instead of talking to her, the player can just take the key to Hjerim off her dead body.
 * After killing Calixto and obtaining the key from his body, the Dragonborn can go in his house. Upstairs in Calixto's chest another Butcher's journal can be found. It tells about the reason of all the killing.
 * Oddly, if you pickpocket the key while Calixto is alive, and steal the journal, that is not enough to get him arrested.
 * When visiting Calixto Corrium (quest step 11), it is possible to sell him the Necromancer's Amulet and then pickpocket the amulet in order to then sell it back. Each successful attempt is rewarded with 500 gold and a high Pickpocket experience (no Pickpocket perks are required). Once Calixto's shop is left, it will remain open for the rest of that day, but will lock and stay locked (Master) from then on. He will not be in there ever again. This only works for one day. You can also loot the key off his dead body after killing him.
 * If you approach the Windhelm Guard with your weapon drawn, the Guard will suspect you of being the killer. However, proclaiming your innocence will be enough to convince them.
 * This quest may be a reference to the Victorian murderer Jack The Ripper.
 * If the corpse is turned into a pile of ashes at the start (using a Raise the Dead Spell), it will "reassemble" back into the regular body when Helgrid takes it into the Hall of the Dead.

Bugs
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Fixed Bugs

 * Patch 1.5 resolved the bug where Calixto did not die properly in some occasions, has been solved.
 * Patch 1.5 fixed the issue where if Viola Giordano is killed before purchasing Hjerim, the quest will start but be in-completable, and no upgrades will be purchasable for the house.

Unpatched Bugs

 * After Dawnguard is installed, Vampires may attack Windhelm at night. If the Windhelm Guard who initiates the quest is killed by the Vampires or their Thralls, the quest cannot be initiated that way. (Needs confirmation)
 * Killing any of the Windhelm guards may cause the quest to never start.
 * Sometimes, after accusing Wuunferth, the body will not appear outside Candlehearth Hall. This can be remedied by talking to Wunnferth (who can be found in jail via the barracks, you can trigger the next part of the quest by choosing "the killer has struck again".)
 * Sometimes after completion of the quest, even if Calixo is caught and Arivanye is saved, she may randomly turn up dead at the stables or around Windhelm.
 * Sometimes after investigating Hjerim, Calixto is untraceable, making it impossible to sell him the amulet. Talking to Viola Giordano still completes the quest.
 * If the player proceeds with the quest by showing the evidence to Viola and speaks to Wuunferth without speaking to Calixto about the amulet, the option to say to Wuunferth "I guess Calixto was wrong" will still appear, and the player will speak of the amulet as through they no longer have it. The quest will still proceed and be possible to complete, but after speaking to Wuunferth, Calixto's house will be locked and he will not be there, making it impossible to sell him the Strange Amulet and thus the Necromancer Amulet cannot be obtained.
 * On some occasions when Calixto runs to Hjerim, after being caught, the marker on the map says that he is outside the city walls, although he is not. If the local map is followed, it indicates his true location (Hjerim).
 * During the second half of the quest, the quest objective to search for the killer in the Grey Quarter at night may be given then immediately completed. Upon leaving the building Calixto will be seen running through the city and if the player attempts to stop him in any way, he will call for help and the Windlhelm Guards will come to his defense. If allowed to escape Calixto will eventually make his way to Hjerim where he will be hiding in the room hidden in the wardrobe, becoming immortal. He will not leave Hjerim and since he cannot be killed the quest will not end.
 * If Civil War Questline has been completed, the mission may be unstartable as you need to be able to talk to a Windhelm Guard and there are only Imperial Guards. I have tried everything else-entering Hjerim (after finishing Mourning Never Comes), reading the Butcher's Journal and Beware the Butcher as well as other things such as talking to the Priestess of Arkay and Nilsine's father and breaking into Calixto's house, taking the third journal-none of it has worked. You are also unable to buy Hjerim as Jorleif (who is still the only one selling Hjerim) says, "There is a house available but there was some...unpleasentness.", however this does not start the quest. Nor does talking to Viola.
 * After chasing Calixto into Hjerim he can't be killed and becomes hostile when you enter.
 * After starting the quest, The quest journal permanantly states "Get Assistance from Jorleif", rendering the quest unfinishable, as Joleif will also bug, so you cannot gain assistance from him.
 * A possible fix for this is to make sure you pick up a butcher flyer and show it to Jorleif, who will then direct you to Viola. As long as you picked up the amulet and showed it to Calixto, you should be able to question Viola about the flyer and then finger Wuunferth.
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