Assassin's Creed Valhalla

Spending your first Mastery point first requires that you reach power level 400, the max level in the game. This may seem daunting at first, but if you're going for all other achievements, you will reach level 400 far before you finish everything else in the game. Once you reach level 400, every time you level up after that you'll receive a Mastery point that you can use in your skill tree. Using your first one unlocks this achievement.
 
You gain XP to level up from completing quests, killing enemies, collecting "collectibles" all around the map (i.e. collecting wealth, and completing mysteries and artifacts on the map), discovering new locations, and syncing viewpoints. You will automatically get 2 skill points for each drengr and legendary animal you defeat, and you will automatically get one skill point each time you complete a standing stone puzzle, a cairn challenge, and an animus anomaly. The vast majority of your XP will come from completing story quests and World Events; you'll only get a small amount for artifacts and wealth collected. But as I said, you'll earn enough XP to reach level 400 well before you finish all of the story quests and complete all of the items in every region. To put it in perspective, by the time you finish all other achievements you'll have leveled up to the equivalent of level 450 or so (giving you, in that case, 50 mastery points to spend).