Some of the Daemons in Oninaki have Shadestone Slots that you can fill and equip to their weapons so you modify and make them more powerful. This Shadestone Guide will tell you everything you need to know about them.
Shadestones are not difficult to find, they can either be picked up as as drops by the various enemies you defeat or found in treasure chests that are randomly found in the different areas of the world. To equip Shadestones that you acquire you will first have to defeat Kushi. This Kushi boss fight will come somewhere in the middle of your story progression of Oninaki. After you have defeated Kushi you will now be able to go to travel to Deto within Szaka from the World Map and select spot #2 to travel to. Upon arrival, head directly to the left and speak to the Alchemist.
If you now select 'Equip Shadestones' a list of all of the weapons you currently have in your inventory will appeary. Select the weapon you want to equip Shadestones to and place how many ever Shadestones that weapon has slots for. As you progress through the game make a point to return and replace older ones Shadestones with ones you find that are superior.
Check out our How to get Soul Stones Guide to find out how they are used to upgrade Daemons to make them more powerful and how you can get them fast.