Once you arrive in the Old Camp in the Gothic Remake, you'll need to beat a handful of quests to complete the Casting Shadows quest for Diego. One such task is The Vanished Warder quest, which you can pick up from Sly near the gate. He'll ask you to find his missing friend Nek, which involves speaking to a bunch of people and eventually tracking down the lost warder.
I'll explain all of the major steps below so you don't have to waste time searching for Nek in the wrong places—very easily done if you haven't immediately purchased a map.
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