While walking through the Grantebridgescire region, you may happen across two children talking about their grandfather: go gate-crash their conversation to find out about him. In this minor world event in Assassin's Creed Valhalla you'll have to go up against a bald, somewhat calm man, who's incredible punches are revered for their power.
The Grantebridgescire Walloper
If you accept his challenge, you must counter all his attacks until his endurance bar is depleted. Once the bar is empty, open him up with a punch of your own and...well, that's it. He'll give you a key to the house on his right, go collect the treasure inside and be on your way.