Very few ponies know the spell to call forth Atherion. Those who do share in his power and he will protect whoever calls upon his might.
Celestia very seldom finds herself in a situation where she must defend herself from those would do her harm. When those times come, Atherion gives his great strength to her gladly.
I wanted to draw this for awhile. Finally got around to it. It would have been out sooner but then computer failure happened! That is fixed though for the time being.
Cute.exe could not be found! Would you like to search again?
Edit: Since there's some confusion, here's some explanation. Atherion is from nowhere. He is simply the tag-along to the drawing idea. He likely won't appear anywhere else of mine.