The Thunderstone is a side quest in the 
Kuttenberg region of Kingdom Come: Deliverance 2.
It involves a long chain of tracking a "magical" artifact through various hands, offering players a choice between rewards or a unique permanent character buff. This quest is very long and requires travelling through the whole Kuttenberg map.
Starting the Quest
The quest begins in the village of 
Grund. Speak with Kona, who can be found near her house (often by the drying racks). She is looking for a protective charm called a Thunderstone to help with her troubles.
She mentions that a man named Thomlin in Horschan currently possesses one, which will set you on a long questline.
Instead, there is a shortcut: you can pick up the Thunderstone directly, skipping the whole quest.
Thunderstone Shortcut
Note: Going for the Thunderstone directly will skip the whole questline and also the task 
Primum Nil Nocere.
Go to Suchdol and enter the house on the southern end. It has a wounded man inside.
On the map, it looks like this:
Find a woman named Mlada. She has the Thunderstone item in her inventory at all times. You can pickpocket it or directly kill her and steal it from her body.
The rest of this guide focuses on the normal, intended way to obtain the Thunderstone.
The Hunt for the Stone
We split this guide into a few steps.
1. Thomlin of Horschan
Travel to Horschan and find Thomlin.
You can challenge him to a wagered duel. This requires passing a hard speech check or having completed the Fight Dirty task beforehand. You must pay a 500 Groschen entry fee.
Do not skip time while he is walking to the barn, or it will mess up the quest.
After beating Thomlin in the fight, he admits he no longer has the real stone. He claims it was stolen by two men, Peter and Hensel, from Raborsch.
2. Peter and Hensel in Raborsch
Head west to Raborsch and speak with Peter.
He reveals that they also lost the stone to a group of mercenaries led by Andrew Ramhead, and that Hensel has been taken captive.
3. The Mercenary Camp
The camp is located northeast of Raborsch along the river. What happens to the bandits does not matter; it is the hostage who has information about the Thunderstone.
Approach the camp and tell the guards you want to speak with Andrew. They will let you in.
With a very high or hard domination check, you can convince him to free Hensel. Alternatively, you can pay 1000 Groschen for his release.
You can also kill Andrew and all his men. This is easiest at night when most are sleeping and unarmed, or during the day, use the rocky outcropping across the river.
Once freed, Hensel reveals the stone was sold to a woman named Mlada in Suchdol.
Hensel can also point you to a hidden stash at the Two Dead Trees interesting site near a cliff alcove containing 357 Groschen. You can stop there before making your way to Suchdol.
4. Mlada in Suchdol
Find Mlada in the southernmost house of Suchdol. She bought the stone to help her sick husband.
She does have the stone and there are two ways to obtain it. First is simply to steal it from her.
Second, the honorable way, is healing her husband, starting the Primum Nil Nocere task. You must complete it to get the stone.
Read the full Primum Nil Nocere guide for diagnosis and healing steps.
Finish the task and she will reward you with the magical stone.
Choosing Your Reward
Once you have the real Thunderstone, you must decide its fate:
- Kona: Alchemy XP, 100 Groschen, and a permanent discount at the Grund Stud Farm.
- Thomlin: Unarmed XP boost or 150 Groschen.
- Yourself: Keep the stone. No gold or XP, but you keep the Thunderstorm Charm buff (see below).
You must visit Kona or Thomlin and then you can choose to keep it for yourself, ending the quest, or give it to them.
You can maximize rewards by completing the turn-in for Kona or Thomlin and then pickpocketing the stone back from them to keep the permanent buff.
We recommend giving it to Thomlin and then pickpocketing him afterward to keep it forever. Thomlin gives more money and while you would get the horse discount if you gave it to Kona, all of their horses are bad and you are unlikely to ever shop there.
This finishes The Thunderstone side quest in Kingdom Come: Deliverance 2.
The Thunderstone Effects
If you keep the stone in your inventory, it grants the following permanent stat bonuses:
- Stamina Regeneration: +10%
- Attack Stamina Regen: +5%
- Block Stamina Regen: +5%
- Stamina Consumption: -10%
The Thunderstone FAQ
Can you skip the full quest?
Yes. Steal or loot the stone from Mlada in southern Suchdol. This skips Primum Nil Nocere.
Best reward setup?
Turn in to Thomlin, pickpocket the stone back, keep the Thunderstorm Charm forever.
How do you duel Thomlin?
Pay 500 Groschen. Pass hard speech or finish Fight Dirty. Do not skip time en route to the barn.
How do you free Hensel?
Hard domination on Andrew, pay 1000 Groschen, or wipe the camp (night is easiest).
What if you keep the stone?
Permanent stamina buffs: +10% regen, +5% attack and block regen, -10% consumption. Also works for Warding Off Evil if you still carry it.