You will receive this quest after first part of the Evil's Soft First Touches quest, when you will be leaving a building in the cutscene. During this quest you will visit the abandoned Bowdon village which is located in the north (M1,8).
After fighting a group of ghouls, you will notice a specific smell near the well. Follow it. The trail will lead you to a building located south. Inside you must examine footprints near the window. The tracks outside will lead you to a haystack in which you will find a body.
Get behind the fence and go north-west where you will find a place from which the victim was shot. Keep following the trail. They will lead you to a bandit camp.
Your target is a note that can be found on the wagon left from the building. After reading it you will learn about the next point to which you must go. There you will find two soldiers. Pick the first dialogue option, this will allow you to make next choices: you can bribe them (1000 crowns), fight them (two enemies on level 32, one of them will have a key) or use Axii sign if you have it developed enough. No matter the choice, now you will be able to get to the hideout. You should consider killing them immediately, as otherwise they will attack you when you will leave the hideout and you will have to fight them anyway. Before the next fight you should definitely use the oil for steel sword.
Inside the hideout of the fallen knights you will face almost ten enemies at once (level 32-34). On high difficulty level or with a low level character/weak equipment this battle might be very hard. It is wise to use some potions (depending on battle style and receipts you have) and many bombs. Trying to stop the enemies (Northern Wind and Samum bombs) and dividing them into smaller groups (Devil's Puffball bomb) would be a good idea.
Finally, go to Oxenfurt, to Alchemy Inn. There you will find the girl that gave you this quest - Adela. For completing the quest, in addition to experience you will receive only 200 crowns.