A command when at war, a livelihood's end, a warning at home, a blacksmith's true friend It rules above clouds, it seethes beneath crust A terrible foe who, in shadows, we trust

First, your brain after a test, or something easy to digest Last, found in every building, but in my case a fairy's dwelling My whole you eat with glee, but sometimes I can poisoned be In tales my ring steals you, or a girl ate me and grew

My first part shines a light on all and makes my second part bloom. I can grown very tall and brighten every garden or room.

If you look inside me very deep, You might find climbers frozen in their sleep. Or maybe you will look and find an ass, But on the surface, see naught but a crack.

I can take many forms, but my purpose is always the same. I can be way up high, or down below, and sometimes even inside. I hold those most precious to us, and some might try to rob me. Some feel free when I am empty. What am I?