To start this quest, talk to Albin Hart, who is on the battlefield not far from the Border Post signpost. This is a kind of support quest for the Fake Papers quest. If you complete this quest and Albin survives, then you will be able to buy a pass from the Shady merchant to cross the border for only 25 coins.
The Nilfgaardian and Redanian forces had fought many bloody yet indecisive battles in Velen from which both sides had to retreat without collecting their dead. On one such field Geralt encountered a group of looters robbing the corpses of the fallen. In another time and place he might have merely grit his teeth and continued on his way. Yet conditions in Velen were harsh and other sources of sustenance few - so he decided to swallow his revulsion and defend these desparate men from the monsters stalking the battlefield.
