so the spell is stored by someone casting the spell into the stone? And then expended by a user?

I was thinking it was more like the pearl of power from 3.5e where it's literally an extra spell slot.

If I can use it while shifted then I'd totally use it(awaiting approval from Jesse and/or official RAW rules somewhere. Having pro evil on me and not taking anyone else's concentration seems like a good strat.

But I'm not going to push that hard for it.