In the Wolfsbane Field – the first Lantern and area that you hit in Chapter 5 of Valor Mortis, you will soon come across a note that begins a short side quest. This can be wrapped up within the current area, and like The Scarecrow or Homesick side quests, it gives a flavour to the struggles of the common people when this war was being waged. This side quest’s path also hands you 2 Ambers.
Here is how to go about starting The Lost Boy side quest, progressing it, and the rewards that you get on completing it in Valor Mortis.
Starting The Lost Boy Quest

Read the note on the bench from the cabin as you move along the main path from the Wolfsbane Field Lantern in Chapter 5. You will come across this place almost the first thing as you proceed down the road from the Lantern. This will be one of the few cabins with the door open, and blood strewn across the steps leading towards it.

The note speaks about a young boy Mirek who is determined to join the rebellion. Reading this note adds this side quest to your quest log. Go to the first floor and listen to the blue Echo as it adds more context.

Unlike Mirek’s promise of returning within a day, it has been 4 days that he has not come back. We then get introduced to Shrike – apparently a group or a God that some villagers, like Mirek’s father, believe to be their protector and salvation.
Progressing The Lost Boy

With this done, the next progress on The Lost Boy side quest happens when you come across the above branching path. You will not miss this, as it is the only way to progress this region. Take the straight path.
The set of crates predictably hide an ambush with a solider and a Volunteer, so approach it carefully. Shoot the Nephtoglobin to unlock a shortcut to the Wolfsbane Field Lantern if you have not done this before.
Listen to the blue Echo – something about carrying Wolfsbane uphill to make The Shrike proud. Prepare for a fight against a Red Eye Sapeur.
Getting the Cellar Key

While his moveset essentially remains the same, the glowing attack slam that he does now has a small fire AoE. The main thing to watch out for, are phantoms that spawn periodically, and throw a flame bomb your way. This can do decent damage as well as cause Burn. It is best to deal with the boss quickly using (Stasis + Blunderbuss).

Defeating him gets you the Equivalent Exchange Amber. Proceed ahead and take the narrow opening on your right to loot a chest with Pure Blade Material, a Cleansing Vial on the rock that you can jump to, and a corpse with the Cellar Key next to it.
Getting the Final Note & Completing the quest

Take the shortcut back, and you can now open the cabin in front of you which was previously locked, with the Cellar Key. This will have The Wound That Heals Amber in the chest on the right, and a note on the table in front.
We learn about cargo that Napoleon is hiding in the camp that the Rebellion is after. Reading this note also concludes The Lost Boy quest in Chapter 5 in Valor Mortis.
