The description on the Wiki says "Claim victory in the Storms of Winter". Then it has four tiers, up to 15 times.
Do you have to complete it 15 times for the mastery point, or do you get the mastery for just doing it once, and extra AP for finishing the achievement?
I spent half of today on Bjora Marches and there was zero interest in this meta. I checked the Wiki page and it is LONG. Completing it 15 times seems like way too much hassle for one mastery point.