The Fifth Commandment is a side quest in Kingdom Come: Deliverance 2 set in the city of 
Kuttenberg.
It unlocks after you complete the main quest Into the Underworld. To begin, speak with Katherine at the Hangman's Halter Tavern. She is worried her contacts are being targeted by a serial killer.
Find the body at the cemetery
Katherine's informant, Alice, has been murdered. Your first job is to investigate the scene and gather clues.
This quest is mostly talking and walking. Dialogues include many skill checks, mostly Scholarship, Speech, and Intimidation.
Examine the body: Go to the cemetery in north Kuttenberg. Find Alice's body in the morgue and use every examine option. Her hair was cut off and her wounds look professional, likely from a sword.
Also speak with Barbora, Alice's friend, who often mourns nearby.
Speak with the locals: Talk to Gravedigger Frantishek near the chapel. Pass a Speech check to learn Alice was not the first victim and that Lumir has been investigating on his own.
Question him further to add an objective to visit the crime scene.
Go to the crime scene (optional): The scene lies northeast of the Hole in the Wall tavern. Examine the blood and use every option.
There is little here beyond blood and slash marks.
Cure Lumir
Find Lumir: He is in a house near the cemetery, but he is badly hungover. Wait a few hours or give him a 
Marigold Decoction (strong quality helps with hangovers) or Hair O' The Dog to wake him immediately.
When he is awake, pass a Presence or Knowledge check to win his help. He names three earlier victims to investigate: Jarka, Stanya, and Zdena.
Investigating the previous victims
Jarka (the servant)
Talk to Father Prokop at the cemetery church. Jarka served the wealthy von Grolle family and was pregnant when she died.
Prokop suspects a minstrel named William, and he resents Lumir for accusing him before. You can pass several checks with Prokop for full context, though it may not add new objectives.
Stanya (the alehouse maid)
Go east in Kuttenberg to the Hole in the Wall inn and speak to Innkeeper Mole.
Pay at least 5 Groschen, then pass an easy Speech check to learn Stanya's room was ransacked. He sends you to charcoal burner Barton.
Before you leave, it is worth starting Zdena's branch too, since both routes leave the city.
Travel to the charcoal burners' camp south of Kuttenberg. Find Barton (blond moustache, green cape) and pass a Speech check. He says Stanya is alive and hiding in a nearby cottage.
If you fail the check, enter one of the southernmost cottages, pick the very easy lock, and talk to Stanya quickly.
Stanya says she escaped an attack. Hendl von Grolle visited her but paid more attention to a “thieving wench” (Zdena, the third victim). Several Speech checks unlock extra lines.
Zdena (the thief)
Speak to Strnad at the barn next to Hole in the Wall. After a fight or a hard Charisma check and 30 Groschen, he says Zdena tried to sell stolen goods to a miller.
Go to Rabstein Mill (south of Bylany) and speak to Pint. He first denies knowing Zdena. Tell him Innkeeper Mole is a bastard and he opens up.
Pint sends you to a ford in the river. At the crossing, look for a basket by a boulder in the water for Zdena's tracks.
If you exhausted Pint's dialogue, you receive Zdena's scarf. With the tracking perk, give it to Mutt and he can lead you to her body.
Otherwise follow the blood trail up the hill to her corpse. Her hand looks broken, as if someone searched for a hidden ring. Return to Kuttenberg.
Identifying the killer
You can question William the Minstrel at the Black Horse Tavern first; he denies involvement.
Return to Lumir and Katherine. Review your notes and pick lines that point at the von Grolle family. Avoid badmouthing Father Prokop.
Every victim ties to von Grolle: Jarka was their servant, Stanya flirted with Hendl, and Zdena robbed them.
If you push blame onto Father Prokop, you can get an objective to murder him. That is not the route you want for a clean outcome.
Infiltrating the von Grolle house
You need hard evidence inside von Grolle manor in central Kuttenberg. Speak to the servant Thaddeus out front. Two main paths:
As a healer: Pass Survival, Scholarship, or Alchemy checks to enter and treat patriarch Gotzlin. You can then speak to mother Mandelina and move through the house.
As a trainer: If you finished Ars Dimicatoria, you can use Menhard's name to get in. Otherwise pass a check to duel Hendl. After you win, tell him to drill his forms to distract him.
Once outside, go upstairs into Hendl's marked room. Lockpick the hard chest beside the bed to find Trophies from Victims: hair locks that match the crime scenes.
The final confrontation
Return to Lumir and set a trap with Katherine as bait. In the cutscene, Katherine pretends to be pregnant with a von Grolle.
The truth: Mandelina (the mother) and Thaddeus (the servant) killed to “protect” Hendl from women they saw as unworthy or dangerous to his name.
Fight Thaddeus: You must defeat him. Afterward you can spare or kill him once dialogue is exhausted.
Deal with Mandelina: Run downstairs. Katherine holds Mandelina at knifepoint. Let Katherine kill her or insist on the bailiff and a trial.
You move to Lumir's house for the closing dialogue.
Rewards: Kuttenberg Knight Shield and 150 Groschen.
The Fifth Commandment FAQ
When does The Fifth Commandment unlock?
After you complete the main quest Into the Underworld. Speak with Katherine at the Hangman's Halter tavern to start.
Who is the killer?
The finale reveals Mandelina von Grolle and the servant Thaddeus as the murderers protecting Hendl's reputation.
Can you fail The Fifth Commandment?
Wrong accusations (such as pushing Prokop) can send you down bad paths. Follow the von Grolle evidence for the intended resolution.
Do you need Ars Dimicatoria?
No. The healer route or dueling Hendl works if you have not finished Menhard's quest line.
What are the ending differences?
Thaddeus can live or die after the fight. Mandelina can die at Katherine's hand or face trial, depending on your choice.