Holy shit they tryna make nobody be able to use magic by making all spirits go into egg form and die and shit
It's Spirit
Liberation Army, not Spirit Extermination Army, the spirits turn into black rock when their mana goes out and they suck all mana from anyone or surrounding area to resurrect/hatch.
People hunt and form contracts with spirits for "power up", they don't need it as Grace was already casting spells before taking a contract with a fire spirit.
If more people will take advantage of spirits that naturally give away mana, people won't cast spells and spirits eventually will turn to black stones, but this time no mana will be around.
That's what I get so far from the story to the point.