Aeterna Noctis gives you a wide range of tools to work with, from weapons like the Spear and Axe to souls which can be used to purchase perks from your skill tree. The King of Darkness can also make use of Power Gems. Once you meet the cartographer along the Route of Souls, you’ll be handed your first one.
With that in your pocket, you might be wondering how to actually use Power Gems — you won’t receive the bonuses they confer unless they’re equipped. This cannot be done on the regular menu, where you’d normally spend skill points. Instead, you need to be sitting on one of the game’s many thrones. Only when the King is seated can he equip Power Gems from the menu, much like how you can only reset your skill tree from the same position.
The King of Darkness has six total slots for Power Gems. He remarks that he likes “three common, one high-powered, and two dark” for himself, and those are indeed the limits of each type you can equip. Power Gems come in those three tiers, and they can boost your damage, increase the chances of a critical hit, and more. Be on the lookout for hidden caves and passages — you never know if you’ll find one inside.