This page of the Assassin's Creed Valhalla trophy guide describes how to unlock the Not the Norse You're Looking For trophy. This achievement is awarded for bypassing a guard in a forbidden zone by blending in with a group of monks.
It is best to unlock the trophy it one of the cities in the game. This can be shown, for example, in a picture of Ledecestre from the Ledecestrescire region in England.
Start by holding down the down direction on the pad. Select Cloak from the drop-down menu.
The cloak makes it more difficult for the opponents to detect you and allows you to "blend in" with your surroundings. Find a group of monks travelling on the streets of Ledecestre. Hold the Triangle / Y to blend in by joining them. Continue to travel with the monks until you pass any guard.