Null Stones in Oninaki are items that can help you unlock the Daemon's skill tree nodes in the Daemon tab of the main menu. This How to get Null Stones in Oninaki will tell you everything you need to know about them.
To level up and upgrade your Daemons so you strengthen them requires two prerequisites that must be met in order to progress. One is you have the required Null Stones and the other is that you have unlocked the nodes of the Daemon's skill tree. Null Stones are not easy to obtain so make sure you pick them up whenever an opportunity arises.
The best way to get Null Stones is to complete the various side quests that are offered to you from the Lost Soul NPC's that are scattered throughout the game. Each of these side quests that you complete will reward you with a Null Sone at the end. These tasks are not hard and can usually be completed in a short amount of time. Check out our How to Complete the Lost Soul Side Quests guide to get more details.
Another way to get Null Stones is to keep an eye out for treasure chests which can be found randomly in the different areas of the game. Not only will you find Null Stones in treasure chests but there is also the possibility of finding Shadestones in them as well. Treasure chests in general will always contain some kind of stone.
Check out our How to get Soul Stones Guide to find out how you can take advantage of an exploit to them fast.
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