At the Mercy of Strangers is a Secondary quest in The Witcher 3. In it, you get a chance to save a Temerian deserter from certain death.
Name: John Verdun
Go west from Mulbrydale, towards the river. You’ll hear screaming – follow it.
Following contains quest spoilers. You’ve been warned
Kill the drowners
Kill the drowners surrounding the tied soldier. He’ll tell you what got him in that mess.