Into the Underworld is the sixteenth main quest in Kingdom Come: Deliverance 2. Following the events of Speak of the Devil, you are tasked with finding Katherine and the rest of the gang.
This mission takes you deep into the city of 
Kuttenberg to locate John of Liechtenstein, who holds information regarding Sir Hans Capon. This main quest runs parallel to The Devil's Pack.
Arrive in Kuttenberg
Your first destination is the Hangman's Halter tavern in Kuttenberg. From the 
Devil's Den, travel southeast.
Along the way, you can stop at Grund to find Bohuta for The Devil's Pack or speak with Karel for the extremely missable task Arrow-Head.
Once you reach the Hangman's Halter tavern near the Kuttenberg central fast travel point, go upstairs to find Katherine. The tavern has a wreath above the door; it is the orange building.
She will tell you that a man named Goatskin has been bragging about spying for a rich lord and spending unusually large amounts of coin.
She sends you to an alehouse called The Hole in the Wall to find him, in the Hoprink district to the east.
Earning Chenyek's respect
As Knuckles, the man near the front door, will tell you, Goatskin is not at the alehouse, so you must speak with a man named Chenyek.
Chenyek is not cooperative at first. You have two options to make him talk.
The nice way: You can pay 200 Groschen or pass a Speech or Intimidation check. Expensive, clean clothes will often make you fail the Speech check; Chenyek aligns more with beggars.
Chenyek may also quiz you with three questions. The correct answers are:
- Repentance (Wycliffe's first thesis)
- The Commoners (the estate with reformatory potential)
- Wenceslas (the rightful king of Bohemia)
The ugly way: If you fail the Intimidation check, you fight a large man named Goliath in the barn behind the inn. You must unequip your armor and weapons for this fistfight.
Whether you win or lose, Chenyek will be impressed by your effort and give you information.
However you get him to talk, Chenyek explains that Goatskin was recently beaten up and usually prowls for targets between Hradek and the Horse Market at night.
Passing this test earns you entry into the rebel guild, and you can ask him for a job. That starts the lengthy Thieves' Guild quest line, beginning with the 
Feast for the Poor side quest, which you can focus on later.
Track down Goatskin and set a trap
You have several ways to locate Goatskin, who is known for robbing German merchants. The game only marks a rough area. You can take the long route through related quests, or go straight to his hideout (see Option 2).
Option 1: long route (intended)
Head to the Black Horse Tavern and speak to Foreman Vlach or Innkeeper Weighman. They will point you toward the bathhouse.
If you want the 
Master Schindel's Toys missable task, this is the moment. Follow the bathhouse steps below in order. Full maps and tunnel route: Master Schindel's Toys guide.
At the bathhouse, first find the bathhouse owner and exhaust all of her dialogue so the Set a Trap objective appears in your journal. Only then speak with bathhouse owner Adam and ask him “What do you need to ‘sort out’?” That starts the Master Schindel's Toys activity. See the Master Schindel's Toys guide for the gallows map, tunnel entrance, and return to Schindel.
Then find Udo at the dice table. Udo is a regular who is often very drunk. You can play dice with him or pass a medium Intimidation check to convince him to leave; in practice the Speech option about his wife is often the most useful.
Goatskin dislikes Germans, and Udo is German. Once he leaves the bathhouse, follow him (it is fine if he sees you).
Go with him. This takes a while because he is very slow. If you are drunk, you can sing with him as you follow. He first goes around the bathhouse, then down the stairs into a dark alleyway.
Then through the All Saints Tavern and more alleyways. Goatskin will eventually ambush him. You can step in and confront him.
Option 2: hideout shortcut
If you want to find Goatskin directly, his hideout is in a barn north of the bathhouse. Note: this skips the Master Schindel's Toys missable task.
You need a bit of parkour. Start at the tailor just southeast of the city fountain, enter the barn, climb the ladder, and cross the upper floor. Drop into the hidden courtyard, climb the carriage, and enter the window of the next barn. If Goatskin is not there, skip time until morning.
When you confront Goatskin, you can use various skill checks and even a 150 Groschen bribe to get information. He will mention a man named Samuel who hired him to spy.
If you have the Master Schindel's Toys task active, you can ask him for more information and continue that task before returning to this guide.
Meeting Samuel and Liechtenstein
The man Goatskin identified, Samuel, can be found near the synagogue in the Jewish Quarter during the day or at the King Solomon Tavern at night.
Samuel will not help immediately. He is suspicious and demands that you infiltrate a meeting of antisemites to prove yourself. Whatever dialogue you pick, the quest advances the same way.
Go to the meeting spot on the eastern side of Kuttenberg marked on the map for 11 PM.
A cutscene plays, with a small plot twist: it is a trap set by Samuel. You are ambushed by two armored fighters.
You do not need to win this fight. A cutscene eventually triggers where Samuel notices the mark on your sword and realizes you are an ally.
Samuel leads you blindfolded to the basement of the King Solomon Tavern to meet John of Liechtenstein, who recognizes you at once. During the conversation, tell him that von Bergow knows his location. Liechtenstein reveals that Sir Hans Capon is likely held at Maleshov Fortress, a property recently gifted to von Bergow.
He suggests seeking the fortress's former owner, Sir Kunzlin Ruthard, to find a way inside. That concludes the quest and begins Via Argentum.
Into the Underworld FAQ
Where do you find Katherine?
Upstairs at the Hangman's Halter, near the Kuttenberg central fast travel point.
Is Chenyek's quiz required?
You can pay or use Speech or Intimidation instead. The quiz is an alternate path if offered.
Should you tail Udo or use the barn?
Tail Udo if you want Master Schindel's Toys and the full trap setup. Use the barn if you want the fastest path to Goatskin.
What if you lose the Samuel ambush fight?
The story still progresses via cutscene; you do not need to win the duel.
What quest follows Into the Underworld?
Via Argentum begins after Liechtenstein points you toward Maleshov Fortress and Sir Kunzlin Ruthard.