Quests are never straightforward in The Witcher 3: Wild Hunt, as they often take unexpected twists and turns, usually ending up being darker or more supernatural than expected; sometimes even both. From the name of this secondary quest, Carnal Sins, one can expect a series of sinister happenings.
Carnal Sins can be started by speaking to Dandelion in The Chameleon (once the establishment that was known as the Rosemary and Thyme) after the previous quest involving this important character, known as Cabaret, has been completed. When the conversation with Geralt's good friend ends, a messenger from the nearby hospital will rush in and inform Dandelion that Priscilla has been attacked and is now receiving treatment. The bard asks Geralt to accompany him and they race to the hospital together to figure out what has happened.
For the most part, this quest is fairly straightforward, though requires some fact-finding and detective work. Be sure to survey each area closely to determine the true villain behind Priscilla's attack and other savage crimes that have happened recently.
Step #1: Meet With The Doctor & Find A Lead
When Geralt and Dandelion get to Vilmerius Hospital, they find that the doctor Joachim von Gratz has stabilized Priscilla and assures the witcher and bard that she will be fine, despite needing a fair bit of time to recover. He then goes on to explain that this has not been the first such attack and that previous victims were not so fortunate.
The doctor suggests that if Geralt wants to find the one responsible, that he should investigate the bodies of the previous victims in the morgue. Naturally, the player's next course of action is to do just that and learn how to proceed from there.
Step #2: Traverse The Sewers & Enter The Morgue
Follow the good doctor into the sewers, though, before heading down, it would be prudent to equip Igni and prepare, as well as apply, some Necrophage Oil to one's silver sword. For in the sewers, one will have to fight a few drowners and a water hag before being able to continue onto the morgue.
After reaching the morgue, Joachim guides Geralt to the body of a dwarf carpenter who was killed in the same manner that Priscilla was attacked. Geralt can investigate the body and gain some insight, though he is interrupted by the coroner, Hubert Rejk, and reverend Nathaniel Pastodi, who both insist that the witcher and doctor leave.
As Geralt and Joachim exit the morgue, the doctor mentions that the reverend used to be an experienced torturer for the Eternal Fire church before being promoted to his current station within the intense faith. This is where the two allies part ways, as Joachim returns to the hospital and Geralt heads off in search of more clues.
Step #3: Run From One Crime Scene To Another To Gather Clues
The next few portions of this quest will involve chasing the quest marker around the map. First, it will lead the player to a prostitute name Whistling Wendy, who will set three thugs to attack the witcher, though they can be defeated easily. Wendy explains, stating that she thought that Geralt was the killer, and apologizes. Investigating the area will allow one to uncover that the culprit seemed to have climbed a very high wall that even Geralt himself could not scale, leading to a suspicion that there is an inhuman element here.
Following this, trail the quest marker to the house of a woodcutter who has also been murdered and speak with the deceased's brother. After the interview, search the yard and the workshop for all possible clues. When this is done, Geralt will have the choice to either speak with the coroner, Hubert, or with Eustace, the corpse collecter. For a bit of bonus XP, go see Eustace first and Hubert second.
When interviewing Eustace, be sure to select the Axii option in the dialogue for 40 XP, if it is successful (the Delusion trait of Axii must be at level 2 or higher). At this, the witcher will learn that there was a note on the woodcutter's mangled body saying that Priscilla would be the next victim. After learning this, head off to see Hubert.
Meet up with the coroner and examine another body, this time both finding another note and learning some suspicious things about Hubert in dialogue with him. The note reads "Patricia Vegelbud", who has not been attacked yet, so Geralt rushes off to intercept the killer and potentially catch them.
Step #4: Almost Catch The Killer
When Geralt reaches the Vegelbud residence, he is met by Ingrid Vegelbud, who lets him in while mentioning Patricia's recent denouncing of the Eternal Fire faith. Sadly, when the witcher finally encounters Patricia, it is too late. However, her killer is close at hand, so the player must give chase.
Unfortunately, the guards will delay Geralt, believing him to be the killer, which will allow the true murderer to get away. Ingrid intervenes, soothing the guards, and allows the White Wolf to see Patricia's body. After investigating the woman's remains, Geralt finds another note, this one pointing to the next victim much the previous ones. Upon it reads the name "Sweet Nettie of Crippled Kate's".
Step #5: Narrow Down The Suspects & Find The Murderer
Upon racing to the brothel and finding Sweet Nettie's room up on the highest floor, burst into it and the player will find reverend Nathaniel next to a bound and gagged Nettie. It is clear from the scene that the reverend was about to begin torturing the young lady. There will be an option to end this scumbag, though the player should verbally confront him first. After doing so, it becomes obvious that although Nathaniel is a violent degenerate, he is not the killer because all evidence points to Hubert Rejk, the coroner. Following this, Nettie will ask the witcher for help, giving Geralt 3 options: do nothing and let the reverend torture Nettie (no reward), use Axii to make Nathaniel leave (40 XP), or kill the reviling reverend (100 XP). There's no doubt that the latter option is both the most lucrative and appropriate.
When leaving the brothel, the player will have enough information for a quest marker to lead Geralt to a warehouse. Within, one will find Hubert, who tells the witcher the details of his plan and his motives in true fashion for a usual villain. After the monologue, he will also reveal that he was a katakan the whole time and attack Geralt! Defeat him with the typical vampire-disposal methods, such as using Igni and Vampire Oil on one's silver sword. The bombs Devil's Puffball and Moon Dust would both be helpful here as well. After the fight, proceed back to the hospital for some good news about Priscilla's injuries not leaving her with any permanent disabilities and a pleasant chat with Dandelion, which will conclude the quest.