Celestia and Luna have ruled in Equestria for over a thousand years. Their age along with the unbelievable power that they wield to move the sun and moon has caused most ponies to see them as gods.
But as powerful as they are, gods they are not. For the being that has arrived in Equestria has powers that can rival and surpass their own. He is a deity millennia of years old, a deity who commands the thunder and lightning with an incredible might and a hammer that can shatter the sun and moon.
And Celestia and Luna will learn that they are not all powerful. That they are not as mighty as they believed. They are not gods.
They are not Thor.