To complete the "Lullaby" quest in Valor Mortis, players must find a way to soothe a crying baby's ghost. This Chapter 3 side quest involves backtracking and can be tricky to navigate, but the rewards are considerable.
How to Start 'Lullaby'
You will encounter the "Lullaby" side quest around the middle of Chapter 3. After looping the Fields Lantern three times and defeating the Volunteer in front of the wagon, proceed to the Marketplace Lantern. From there, turn left toward the river and enter the house on your left. Investigating the crib in the first room will initiate the quest.

Completing 'Lullaby'
With the quest started, exit the baby's house through the back door. Defeat the two civilian zombies in the backyard and pick up the Weathered Key from the ground behind them. Return to the Marketplace Lantern, pass through the wagon, and proceed to the right. If the Volunteer has respawned, it is advisable to attack from inside the wagon to avoid direct engagement. Alternatively, you can teleport to the Fields Lantern and take the path through the hayfields to bypass the Volunteer.

Cross the river by jumping across the platforms on the other side of the hamlet. Now, use the Weathered Key to enter the house.

Collect the toy from inside the house. Afterward, return to the crib with the crying baby's ghost and place the toy inside. This action will complete the "Lullaby" quest, rewarding players with 1,500 catalysts and the Tomorrow's Promise amber.