Since I haven't done much in a while, I thought you might want a spoiler and some info on the Devonstone.
The Devonstone is essentially a large source of adrenaline to put it in perspective. When the effects wear off, Kelious's body takes extra toll. Hence why he must find out what power it actually contains. Also, with Davina being put into a coma is a demonstration of this which I didn't explain properly. Kelious was born to endure the Stone's power, but Davina is not, hence why she took a much larger toll when being healed by the Devonstone.
Also, for those of you who were wondering why the Devonstone has more than one ring floating in the center, is because they aren't rings. They are actually symbols of each different song. The Devonstone itself is a device used to harness the power of all them combined, with an infused DNA code to unlocking it. The reason why the Devonstone isn't recorded in history, is because there was no knowledge of its existence. It dissapeared along with the Devonoris clan, presumably to safeguard it. When/If he meets Galleous, they will research how to unlock the said device, releasing the individual Songs.
Hope that gives you some theories. Be sure to share those with me below! :)
Dr Playz