Power Stone is a Relic.
How to Get Edit
Beat the relevant mission in The Tale of Horus Traver
Created By Edit
This is a more reliable way to ramp in Eternal, as relic removal is limited, and units with similar effects, such as Avirax Familiar and Initiate of the Sands are weak to the many forms of unit removal.
When Power Stone is removed from play, you lose the Maximum Power.