This quest is found at an old castle a short ride to the west of Kaer Morhen. It is a short quest that doesn't involve travelling outside the old castle.
Upon arrival, the quest will be triggered automatically. The Witcher will find four level 23 Wraiths, which he can optionally kill.
After a short walk around, Geralt will comment to use Keira's Lamp. A few short cutscenes of a harsh Witcher instructing a boy recruit can be found around the inside of the castle. Afterwards, climb the stairs to the ladder leading up to the eastern wall of the Bastion, and follow the battlements to the northeast tower.
At the top, a final scene can be viewed via the lamp. A skeleton pinned to the wall with pitchforks can be looted for a young boys bones, which must be buried down below on ruines room adjoining the western wall. Head back down to the ground level. CAUTION; The ladder can be hard to access leading the witcher to fall a great height. A quicksave and ensuring you're at full health is recommended.
Four new wraiths will appear, attempting to stop you. Deal with them and bury the bones.