Assassin's Creed Odyssey is, much like Origins, a fully-fledged action RPG instead of being an action-adventure title like the earliest iterations of the franchise. This means that with it come all the usual features of RPGs, one of which is levelling, too.
Assassin's Creed Odyssey max level is not something you can reach in a few days' worth of casual gameplay, but if you've been with the game since release and have been keeping track of Ubisoft's numerous post-launch DLC, it's only a matter of time before you hit level cap. And what then?
Assassin's Creed Mastery Levels are what you get to fiddle with once you've reached Alexios' or Kassandra's max level cap. Separated into three sets of sills: Hunter, Warrior, and Assassin, these Mastery Levels allow for substantial and situational min-maxing of your character build.
There are 72 Mastery Levels in total, with each allowing for 20 upgrades, each of which costs 1 ability point. What this means is that, to fully max-out your Mastery Levels, you're going to need exactly 1440 ability points after you've already reached your level cap. How's that for endgame goals?
Best thing about Mastery Levels is that you can respec your build whenever you so desire, allowing you to rebuild your passives however you see fit. Since Mastery Levels stack with all your other weapon, armour, engravings, and character stats, you're in for quite a treat when you finally open them up.