When you first get off the airship and arrive at the Valley of Dead Heroes, you'll be approached by three cultists who tell you in no uncertain terms that you're going to help them find some people they're trying to track down.
Resist the urge to attack or intimidate them, and tell them you'll help (you have the option to lie when you do, but I'm unsure if that actually matters for this).
Then check the boarded up alcoves opposite the stairs the three approached from. One of them will prompt a perception check. FAIL this check, and return to the cultists to report your failure.
Since you were actually unable to find them, they'll decide you're telling the truth and leave. Soon after, the achievement will unlock.